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TEXT-BOOK OF GEOLOGY 

/ 






BY 



SIR ARCHIBALD GJIIKIE, F.R.S. 

D.Sc. Oaiib., Dubl. : LL.D. Edin., St. And. 

MBfX/nill-UEIIKRAL OF THK OKOUKIK'AL 8UKVBY Or OIUUT BRITAIN AND IRELAMD 

CORRKSPOXDCrr or THE INHTITtrrE OP PRANCE 

CORBI^PONDKNT OP THE ROTAL ACADEMY OF HCIKXCE8 OP BBRLIK 

Ac, &r. 



THIRD EDITION, REVISED AND ENLARGED 



i^onlion 
MACMILLAN AND CO. 

AND NEW YORK 
1893 

t 
-I" 

AU righU natrvtd 



2ii 183 



Firtt Edition, i88a ; Second, 1885 ; Third, 1893. 









, • • • • 



PREFACE TO THE THIRD EDITION 

The present edition of this Text:book has been entirely revised, and in 
some portions recast or rewritten, so as to bring it abreast of the 
continuous advance of Geological Science. The additions made to the 
text, which extend to every branch of the subject, increase the volume 
by about 150 pages. Care has been taken to preserve a characteristic 
feature of former editions by inserting references to the more important 
memoirs and papers, where the student will find fuller information than 
can be given in a Text-book, 

While the book was passing through the press I received from my 
friend Prof. Zirkel the first volume of the new edition of his great text- 
book of Petrography, but too late to avail myself of its assistance. I can 
only now recommend it as an indispensable part of the outfit of every 
serious student of the petrographical section of Geology. 

In the revision of the stratigraphical portion of this work I have 
been assisted with suggestions and information by my colleagues, Mr. 
Topley, Mr. H. B. Woodward, Mr. E T. Newton, and Mr. C. Reid, to 
whom my best thanks are due. 

Museum, Jermyn Street, 
1^ August 1893. 



FROM THE PREFACE TO THE FIRST EDITION 



The method of treatment adopted in this Text-book is one which, while 
conducting the class of Greology in the University of Edinburgh, I have 
found to afford the student a good grasp of the general principles of the 
science, and at the same time a familiarity with and interest in details of 
which he is enabled to see the bearing in the general system of know- 
ledge. A portion of the volume appeared in the autumn of 1879 as the 
article " G^logy " in the Encydopxdia Britannica, My leisure since that 
date has been chiefly devoted to expanding those sections of the treatise 
which could not be adequately developed in the pages of a general work 
of reference. 

While the book will not, I hope, repel the general reader who cares 
to know somewhat in detail the facts and principles of one of the most 
fascinating branches of natural history, it is intended primarily for 
students, and is therefore adapted specially for their use. The digest 
given of each subject will be found to be accompanied by references to 
memoirs where a fuller statement may be sought. It has long been a 
charge against the geologists of Great Britain that, like their countrymen 
in general, they are apt to be somewhat insular in their conceptions, even 
in regard to their own branch of science. Of course, specialists who have 
devoted themselves to the investigation of certain geological formations 
or of a certain group of fossil animals, have made themselves familiar 
with what has been written upon their subject in other countries. But I 
am afraid there is still not a little truth in the charge, that the general 
body of geologists here is but vaguely acquainted with geological types 
and illustrations other than such as have been drawn from the area of the 
British Isles. More particularly is the accusation true in regard to 
American geology. Comparatively few of us have any adequate concep- 
tion of the simplicity and grandeur of the examples by which the principles 
of the science have been enforced on the other side of the Atlantic. 

Fully sensible of this natural tendency, I have tried to keep it in 
constant view as a danger to be avoided as far as the conditions of my 
task would allow. In a text-book designed for use in Britain, the illustra- 
tions must obviously be in the first place British. A truth can be enforced 



viii TEXT-BOOK OF GEOLOGY 

much more vividly by an example culled from familiar ground than by 
one taken from a distance. But I have striven to widen the vision of the 
student by indicating to him that while the general principles of the 
science remain uniform, they receive sometimes a clearer, sometimes a 
somewhat diflferent, light from the rocks of other countries than our own. 
If from these references he is induced to turn to the labours of our fellow- 
workers on the Continent) and to share my respect and admiration for 
them, a large part of my design will have beei^ accomplished. If, further, 
he is led to study with interest the work of our brethren across the 
Atlantic, and to join in my hearty regard for it and for them, another 
important section of my task will have been fulfilled. And if in perusing 
these pages he should find in them any stimulus to explore nature for 
himself, to wander with the enthusiasm of a true geologist over the length 
and breadth of his own country, and, where opportunity offers, to extend 
his experience and widen his sympathies by exploring the rocks of other 
lands, the remaining and chief part of my aim would be attained. 

The illustrations of Fossils in Book VI. have been chiefly drawn by 
Mr. George Sharman ; a few by Mr. B. N. Peach, and one or two by Dr. 
R. H. Traquair, F.R.S., to all of whom my best thanks are due. The 
publishers having become possessed of the wood-blocks of Sir Henry de 
la Beche's * Geological Observer,' I gladly made use of them as far as they 
could be employed in Books III. and IV. Sir Henry's sketches were 
always both clear and artistic, and I hope that students will not be sorry 
to see some of them revived. They are indicated by the letter {B), The 
engravings of the microscopic structure of rocks are from my own draw- 
ings, and I have also availed myself of materials from my sketch-books. 
The frontispiece is a reduction of a drawing by Mr. W. H. Holmes, whose 
pictures of the scenery in the Far West of the United States are by far 
the most remarkable examples yet attained of the union of artistic 
effectiveness with almost diagrammatic geological distinctness and accuracy. 
Captain Dutton, of the Geological Survey of the United States, furnished 
me with this .drawing, and also requested Mr. Holmes to make for me 
the caiion-sections given in Book VII. To both of these kind friends I 
desire to acknowledge my indebtedness. 



CONTENTS 



PAGE 

INTRODUCTION 1 

BOOK L 
CosMiCAL Aspects of Geology, 7. 

I. Relations of the Earth in the Solar System ... 8 

II. Form and Size of the Earth ...... 13 

III. Movements of the Earth in their Geological Relations 15 

1. Rotation, lb— 2. Revolution, 16—3. Precession of the Equinoxes, 16 — 4. Change 
in the obliquity of the Ecliptic, 17—5. Stability of the Earth's Axis, 17—4). 
Changes of the Earth's Centre of Gravity, 20—7. Results of the Attractive 
Influence of 3un and Moon on the Geological Condition of the Earth, 21—8. 
Climate in ito Geological Relations, 28. 



BOOK II. 

Geognosy — An Investigation of the Materials of the 

Eariu's Substance. 

Part I. — A General Description op the Parts of the Earth. 
I. The Envelopes — Atmosphere and Hydrosphere . .31 

1. The Atmosphere, 81—2. The Oceans, 83. 

II. The Solid Glohe or Lithosphere ..... 38 

1. The Outer Surface, 38—2. The Crust, 46—3. The Interior or Nucleus, 47; 
Evi<lence of Internal Heat, 48 ; Irregularities In the downward Increment of 
Heat, 51 ; Proljable Condition of the Earth's Interior, 5»— 4. Age of the Earth 
and Measures of Geological Time, 58. 

Part II. — An Account op the Composition op the Earth's Crust — 

Minerals and Rocks. 

I. General Chemical Constitution of the Crust . .60 

II. Rock-forming Minerals ....... 64 

III. Determination of Rocks . .80 

I. Megascopic Examination, 81— ii. Chemical Analysis, 87— iii. Chemical Synthesis, 
89— iv. Microscopic Investigation, 80. 

IV. General Outward or Megascopic Characters of Rocks . . 96 

1. structure, 96—2. Composition, 104—3. State of Aggregation, 105 — 4. Colour and 
Lustre, 106—6. Feel and Smell, 107— <5. Si)ecitlc Gravity, 108—7. Magnetism, 108. 



TEXT-BOOK OF GEOLOGY 



PAOE 

V. Microscopic Characters of Rocks ..... 108 

1. Microscopic Elements of Rocks, 109 — 2. Microscopic Structures, 117. 

VI. Classification of Rooks ....... 123 

VII. A Description of the more important Rocks of the Earth*s Crust. 
i. Sedimentary 



A. FragmetUal {Clastic) 
1.* Gravel and Sand Rocks (Psammites) 

2. Clay Rocks (Pelites) . 

3. Volcanic Fraginental Rocks (Tuffs) 

4. Fragmental Rocks of Organic Origin 

(1) Calcareous, 188— <2) Siliceous, 141— (8) Phospliatic, 141— (4) Carbonaceous, 
142— (6) Ferruginous, 140. 



126 

126 
127 
132 
135 

138 



B. Crystalline^ including Bocks formed from Chemical Precipitation 148 
ii. Massive, Eruptive, Igneous . . . . . .154 

i. Acid Series, 156— ii. Intermediate Series, 163— iii. Basic Series, 169. 

iii. Schistose (Metamorphic) . . . . . . .175 

1. Argillites, 179—2. Quartz-Rocks, 179—8. Pyroxene-Rocks, 181 — 1. Hornblende- 
Rocks, 182—5. Garnet-Rocks, 182—6. Epidote-Rocks, 188—7. Chlorite-RockM, 
183—8. Talc-Rocks, IKS— 9 OH vine-Rocks, or Peridotites, 183—10. Felsitoid- 
Rocks, 183—11. Quartz- and Tourmaline- Rocks, 184 — 12. Quart-z- and Mica- 
Rocks, 184 — 13. Quartz- and Felspar-Rocks, 185 — 14. Quartz-, Felspar-, and 
Mica-Rocks, 185 — 15. Quartz-, Felspar-, and Oamet-Ro<'.ks, 186 — 16. Felsiwr- 
and Mica- Rocks, 187 — Composition of some Schistose Rocks, 188. 



BOOK III. 

Dynamical Geology, 189. 

Part I. — Hypoqene Action — ^An Inquiry into the Geological Changes 
IN Progress beneath the Surface of the Earth, 190. 

I. Volcanoes and Volcanic Action. 

1. Volcanic Products ........ 191 

I. Oases and Vajwurs, 193—2. Water, 197—3. Lava, 198—4. Fragmentary Mate- 
rials, 199. 

2. Volcanic Action ........ 202 

Active, Dormant, and Extinct Phases, 202 — Sites of Volcanic Action, 203 — 
Onlinary Phase of an Active Volcano, 204 — Ck>nditions of Eruption, 205 — 
Periodicity of Eruptions, 206 — General Sequence of Events in an Eruption, 
207— Fissures, 208— Explosions. 211— Showers of Dust and Stones, 218— Lava- 
streams, 217— Elevation and Subsidence, 231— Torrents of Water and Mud, 282 
—Effects of the closing of a Volcanic Chimney— Sills and Dykes, 238— Ex- 
halations of Vapours and Gases, 233— Geysers, 235— Mud Volcanoes, 238. 

3. Structure of Volcanoes . . ..... 239 

i. Volcanic Cones, 240 — Submarine Volcanoes, and Volcanic Islands, 249 — ii. 
Fissure (Massive) Eruptions, 255. 

4. Geographical and Geological Distribution of Volcanoes . 259 

5. Causes of Volcanic Action ....... 263 

II. Earthquakes ........ 270 

Amplitude of Earth - Movements, 271— Velocity, 272— Duration, 273— Modifying 
Influence of Geological Structure, 273— Extent of Country affected, 274— Depth 
of ijource, 275— Geological Effects, 276— Distribution, 279— Origin, 280. 



CONTENTS xi 



PAOB 

III. Secular Upheaval and Depbebsion 281 

Upheaval, 284— Sabaidence, 28S— Causes of Upheaval and Snbaidence, 292. 

IV. Hypooene Causes of Changes in the Texture, Structure, and 

Composition of Rooks ....... 296 

1. Effects of Heat ....... 297 

Rise of Temperature by Depression, 297— Rise of TempeFatore by Chemical Trans- 
formation, 298— Rise or Temperature by Rock-cnishing, 298— Rise of Tempera- 
ture by Introsion of Erupted Rock, 299— Expansion, 299— Crystallisation 
(Marble), 800— Production of Prismatic Structure, 800-Dry Fusion, 800— 
Contraction of Rocks in passing fh)m a Glassy to a Stony State, SOi— Sublima- 
tion, 805. 

2. Influence of Heated Water ....... 305 

Presence of Water in all Rocks, 806— Solvent Power of Water among Rocks, 807— 
This Power increased by Heat, 807— Co-operation of Pressure, 807- Aquo- 
igneous Fusion, 808— Artificial Production of Minerals, 809— Artificial Altera- 
tion of Internal Structures, 809. 

3. Effects of Compression, Tension, and Fracture . . . .311 

Minor Ruptures and Noises, 811— Consolidation and Welding 812— Cleavage, 312— 
Deformation, 814— Plication, 817— Jointing and Dislocation, 318. 

4. The Metamorphism of Rocks ...... 319 

Production of Marble from Limestone, 820— Dolomitisation, 821— Conversion of 
Vegetable Substance into Coal, 822— Production of new Minerals, 8^— Pro- 
duction of the Schistose Structure, 328. 



Part II. — Epioene or Surface Action, 325. 

I. Air ... . 326 

1. Geological Work on Land ....... 327 

(1) Destructive Action, 327— Effects of Lightning, 828— Effects of Changes of Tem- 
perature, 328— Effects of Wind, 829— (2) Reproductive Action— Growth of Dust, 
SSl— Loess, 882— Sand -Hills or Dunes, 834— Dust -showers. Blood-rain, 837 
—Transportation of Seeds, 838— Efflorescence Products, 388. 

2. Influence on Water ........ 338 

Ocean Currents, 388— Waves, 389— Alteration of Water-level, 839. 

IL Water ......... 339 

1. Rain ......... 341 

(1) Chemical Action, 841 — Chemical Composition of Rain-water, 841— Chemical and 
Mineralogical Changes produced by Rain, 843— Weathering, 845— Formation of 
Soil, 851 -(2) Mechanical Action, 853— Removal and Renewal of Soil, 853— 
Movement of Soil-cap, 854 — Unequal Erosive Action of Rain, 354. 

2. Underground Water ....... 35^ 

Springs, 357— (1) Chemical Action, 860— Alteration of Rocks, 8d4 — Chemical 
Deposits, 865 — Subterranean Channels and Caverns, 367 — (2) Mechanical 
Action, 369. 

3. Brooks and Rivers ........ 371 

i. Sources of Supply, 871— ii. Discharge, 873— iii. Flow. 375— iv. Geological Action, 
877: (1) Chemical, 877; (2) Mechanical, 379 — Transporting Power, 879— 
Excavating Power, 384 — Reproductive Power, 893 — Cones de Ejection, 393 — 
River-beds, 894 — Flood-plains, 895— Deposits in Lakes, 397— Bars and Lagoon- 
barriers, 898— Deltas in the Sea, 400— Sea-borne Sediment, 408. 

4. Lakes ......... 404 

Fresh-water, 404— Saline, 408— Deposits in Salt and Bitter Lakes, 411. 

5. Terrestrial Ic» ........ 413 

Frost, 418— Froren Rivers and Ijikes, 414— Hail, 415— iJnow, 416— Glaciers and Ice- 
SheeU, 417— Work of Glaciers : (a) Transport, 423 ; (6) Erosion, 427. 



xii TEXT-BOOK OF GEOLOGY 

PAGE 

6. Oceanic Waters ........ 482 

i. Movements : (1) Tides, 488— (2) Corrents, 4S4— (3) Waves and Groond-Kwell, 436 
— <4) Ice on the Sea, 488— ii. Geological Work : (1) Influence on Climate, 440— 
(2) Erosion: (a) Chemical, 441; (b) Mechanical, 442— (3) Transport, 450— 
i4) Reproduction, 453 — Chemical Deposits, 453 — Mechanical Deposits, 454: 
(a) Land-derived or Terrigenous : Shore Deposits, 454; Inftn-littoral and Deeper- 
water Deposits, 455— <&) Abysmal or Pelagic, 457. 

7. Denudation and Deposition — The Results of the Action of Air and Water 

upon Land ........ 460 

i. Subaerial Denudation : the general Lowering of Land, 460 — 2. Subaerial Denuda- 
tion : the unequal Erosion of Land. 465 — 3. Marine Denudation, its compara- 
tive Rate, 466 — 4. Marine Denudation, its final Result, 468—5. Deposition: 
the Framework of New Land, 470. 

III. Life. 

1. Destructive Action of Plants and Animals ..... 471 

2. Conservative Action . . . .475 

3. Reproductive Action . . . . . .477 

Sea-weeds, 477— Humus and Black Soils, 477— Peat-Mosses and Bogs, 478 — Man- 
grove Swamps, 481 — Diatom Earth, 481— Chemical Deposits formed by Plant- 
agency, 482- Chemical Deposits formed by Animal-agency, 484 — Shell-marl, 
484 — Coral-reefs, 485— Limestone and Ooze, 492— Siliceous Ooze, 493— Phos- 
phatic Deposits, 494. 

4. Man as a Geological Agent ....... 495 



BOOK IV. 

Gkotectonic (Structural) Geology, or the Architecture of 

THE Earth's Crust. 

Part I. — Stratification and its Accompaniments, 498. 

Forms of Bedding. 498— False-bedding, 501— Intercalated Contortion, 502— Irr^u- 
laritics of Beading due to Inequalities of Deposition or of Erosion, 504 — Sur- 
face-Markings (Ripple-mark, Sun-cracks, kc), 507— Concretions, 510 — Alterna- 
tions and Associations of Strata, 513— Relative Persistence of Strata, 515 — 
Influence of the Attenuation of Strata upon apparent Dip, 517 — Overlap, 518 — 
Relative Lapse of Time represented by Strata and by the Intervals between 
them, 518 — Ternary Succession of Strata, 521— Groups of Strata, 522— Order of 
Superposition : the Foundation of Geological Chronology, 523. 

Part IL — Joints, 523. 

1. In Stratified Rocks, 524—2. In Massive (Igneous) Rocks, 527—3. In Foliated 
(Schistose) Rocks, 531. 

Part IIL — Inclination of Rocks, 531. 

Dip, 581— Outcrop, 533— Strike, 534. 

Part IV. — Curvature, 536. 

Monoclines, 638 — Anticlines and Synclines, SSS^Invcrsion, 530— Crumpling, 541— 
Deformation and Crushing, 543. 

Part V. — Cleavage, 545 
Part VI. — Dislocation, 547. 

Nature of Faults, 548— Origin of Faults, 550— Normal Faults, 550— Reversed Faults, 
550— Thrust-planes, 551— Throw of Faults, 551— Dip- Faults and Strike-Faults, 
552— Dying out of Faults, 555 — Groups of Faults, 556 — Detection and Tracing 
of Faults, 557. 



CONTENTS xiii 



Part VIL — Erdptivb (Igneous) Rocks as Part op the Structure op the 

Earth's Crust, 559. 

PAGE 

I. Plutonic, Intrusive, or Subsequent Phase of Eruptivity . 663 

1. Bosses ......... 564 

Granite-boMes, 565 — Relation of Granite to Contiguous Rocks, 567— Connection of 
Granite with Volcanic Rocks, 569— Diorite, &c, 571— Enects on Contiffuous 
Rocks, 572— Connection with Volcanic Action and with Crystalline Schists, 
573. 

2. Sheets, Sills ........ 573 

General Characters, 578— Effects on Contiguous Rocks, 676— Connection with Vol- 
canic Action, 576. 

3. Veins and Dykes ........ 577 

Eruptive or Intrusive, 678— " Contemporaneous " and other Veins, 580— Dykes, 582 
— Effects on Contiguous Rocks, 584. 

4. Necks ......... 584 

II. Interbedded, Volcanic, or Contemporaneous Phase of Eruptivity. 

1. Crystalline, or Lavas ....... 589 

2. Fragmental, or Tuffs ....... 598 

Part VIII. — Metamorphism, Local and Regional, 595. 
I. Local Metamorphism (Metamorphism of Contact or Juxtaposition) 597 

Bleaching, 598— Coloration, 5f)8— Induration, 508— Expulsion of Water, 599— Pris- 
matic Structure, 599— Calcination, Melting, Coking, 600— Marmarosis, 602— 
Production of New Minerals, 608 — Production of Foliation, 604 — Alteration of 
the Intrusive Rock, 608— Summary of Facts, 609. 

II. Regional (Normal) Metamorphism, the Crystalline Schisi*8 611 

Introduction : General Characters of the Crystalline Schists, 611— Dispute regarding 
their Origin, 618 — Influence of Movements of the Earth's Crust, 614— Nature of 
the rock-changes in Regional Metamorphism, 617 — Illustrative Examples : 
Anlennes, 619— Taunus, 620— Scandinavia, 621— The Alps, 622— Scottish High- 
lands, 624— Greece, 627— Green Mountains, 628 — Menominee and Marquette 
Regions, 628— Table showing the wide Range of Geological Systems affected by 
Regional Metamorphism, 628— Summary, 629. 

Part IX. — Ore Deposits, 631. 

i. Mineral- Veins or Lodes, 638— Variations in Breadth, OSS— Structure and Con- 
tents, 6S4— Successive infilling, 636— Connection with Faults, 686— Relation of 
Contents to Surrounding Rocks, 688 — Decomposition and Recom position, 638. 

ii. Stocks and Stock -works, 6S9— Origin of Mineral- Veins, 640. 

Part X. — Unconformability, 641. 

BOOK V. 
Pat^ontological Geology, 645. 

Deflnition of the term Fossil, 645. i. Conditions for the Entombment of Organic 
Remains, 646: on Land, 646; in the Sea, 648— ii. Preservation of Organic 
Remains in Mineral Masses, 650—1. Influence of Original Structure and Com- 
position, 650—2. FossllisatiJm, 651— iii. Relative Palseontological Value of 
Organic Remains, 652— iv. Uses of Fossils in Geology, 658. They show H) 
Changes in Physical Geology, 663 ; (2) Geological Chr(»noli>gy, 656 ; (S) Sub- 
divisions of the Geological Ueconl, 661— v. Bearing of Palteontological Data 
upon Evolution, 666— vi. The Collecting of Fossils, 669. 



XIV 



TEXT-BOOK OF GEOLOGY 



BOOK VI. 



Stratigraphical Geology. 



General Principles 



I'AOE 

674 



Table of the Stratified Formations constituting the Geological Record — 

TofoM p. 679 



Part I. — Pre-Cambrian. 

m 

i. General Characters ........ 

1. The lowest Gneisses and Schists ..... 

2. Pre-Cambrian Sedimentary and Volcanic Groups 

ii. Local Development ....... 

Britain. 698— ScaDdinavia, 711— Central Euroiie, 713— America, 715— India, 717 - 
China, 717— Australasia, 717. 



680 
685 
692 
698 



Part II. — PALiRozoic, 718. 

L Cambrian (Primordial Silurian). 

1. General Characters, Rocks, Flora and Fauna .... 

2. Local Development ........ 

Britain, 725— Continental Enrojte, 781 — North America, 735— South America, China, 
India, Australia, 787. 



719 
725 



n. Silurian 

1. General Characters . 

2. Local Development . 



737 

73. 
746 



Britain, 746— Basin of the Baltic, Russia and i^candinavia, 766— Western Europe, 
7<W— Central and Southern Europe, 772— North America, 775— Asia, 776— 
Australasia, 776. 

IIL Devonian and Old Red Sandstone .777 

(i.) Devonian Type, 

1. General Characters ........ 778 

2. Local Development. ....... 783 

Britain, 788— Central Europe, 785— Russia, 788--Nortli America, 7S9— Aaia, 790— 
Australasia, 790. 

(ii.) Old Bed Sandstone Tyjtc. 

1. General Characters ........ 791 

2. Local Development ........ 797 

BriUin, 797— Norway, Ac, 802— North America, 803. 



IV. Carbonikerous. 

1. General Characters . 

2. Local Development . 



British Isles, 824— Franco and Belgium, 834— Gennany, 836-SouU\em Gennany, 
Bohemia, H37— Alus, Italy, 838— Russia, 838— Spitzbergeu, 338— Africa, 839— 
Asia, 839— Australasia, 839— North America, 840. 



804 
824 



CONTENTS 



XV 



V. Permian (Dyas). 

1. General Characters . 

2. Local Development . 



BriUin, 840— GeniunT, Ac, 848— Voages, 860— France, 851— Alps, 852— Russia, 
852— Asia, 853— Australia, 854— Africa, 855 — North America, 855— Spitz- 
bergen, 856. 



PAGE 

841 
846 



Part IIL — Mesozoic or Secondary, 856. 

I. Triassic. 

1. General Characters ........ 

2. Local Development. ....... 

BriUin, 864— Central Europe, 868— Scandinavia. 870— Alpine Trias, 871- Spitz- 
bergen, 876— Asia, 877— Australia, 877— New Zealand, 878— Africa, 878— North 
America, 878. 

IL Jurassic. 

1. General Characters ........ 

2. Local Development. ....... 

Britain, 897— France and the Jure, 910— Oennany, 915— Alps, 917— Sweden, 918— 
Russia, 918— North America, 919— Asia, 919— Austrelasia, 92a 

IIL Cretaceous. 

1, General Characters ........ 

2. Local Development ........ 

Britain, 936— France and Belgium, 947— Germany, 953— Switzerland and the Alps, 
954— Basin of the Mediterranean, 956— Russia, 956— India, 957— North America, 
957— Australasia, 960. 



858 
864 



879 
897 



920 
986 



x' 



Part IV. — Cainozoic or Tertiary, 961. 



I. EocENi-:. 

1. General Characters ........ 

2. Local Development ........ 

Britain, 970— Northern France and Belgium, 975— Southern Euroi>e, 979— India, 
&c., 981— North America, 981— Australasia, 982. 

11. Olkjocene. 

1. General Characters ........ 

2. lx)cal Development. ....... 

Britain, 986— France, 989— Belgium, 990— Germany, 991— Switzerland, 992— Vienna 
Basin, 992— Italy, 998— North America, 993. 

111. Miocene. 

1. (leneral Characters ........ 

2. Local Development ........ 

France, 998— Belgium, 998— Germany, 999— Mainz Basin, 999— Vienna Basin, 999— 
SwltzerUnd, 1000- Italy, 1001— Greenland, 1001- India, 1002— North America, 
1002— Australia, 1008— New Zealand, 1003. 



IV. Pliocene. 

1. General Characters . 

2. Local Development . 



Britain. 1008— Belgium and Holland, 1015— France, 1015— lUly, 1016 -Gennany, 
1017— Vienna Basin, 1018— Greece, 1019— Samos, 1020— India, 1020— North 
America, 1022— Australia, 1022— New Zealand, 1023. 



964 
970 



983 
986 



993 
998 



1003 
1008 



xvi TEXT'BOOK OF GEOLOGY 

Part V. — Post-Tertiary or Quaternary, 1023. 
I. Pleistocene or Glacial. 

PAGE 

1. General Characters ........ 1024 

Pre-fflacial Land-surfaces, 1025— The Northern Ice-sheet, 102G— loe-crumpled 
Rocks, 1031— Detritus of the Ice-sheet, Boulder-clay, Till, 1031— Inter-glacial 
beds, 1038— Evidences of Submergence, 1036— Second Glaciation, Re-elevation, 
Raised Beaches, 1037. 

2. Local Development ........ 1042 

Britain, 1042— Scandinavia, 1045— Ocrniany, 1045— France, 1046— Belgium, 1047— 
Alps, 104&— Russia, 1049— North America, 1050— India, 1054— Australasia, 1065. 

II. Recent, Post-glacial or Human Period. 

1. General Characters ........ 1055 

Palffiolithic Alluvia, 1057— Brick-Earths, 1058— Cavern Deposits, 1058— Calcareous 
Tufks, 1059— Loess, 1059— Pabcolithic Fauna, 1061— Neolithic, 1063. 

2. Local Development ........ 1065 

Britain, 1065— France, 1065— Germany, 1066— Smteerland, 106d— Denmark, 1066— 
North America, 1067— Australasia, 1067. 



BOOK VII. 
Physiographigal Geology, 1068. 

1. Terrestrial Features due more or less directly to Disturbance of the Crust, 1071 
—2. Terrestrial Futures due to Volcanic Action, 1079—8. Terrestrial Features 
due to Denudation, 1079. 



List of Authors quoted or referred to . . 1091 

■ 

Index .... ..... 1103 



* . 



• -. 



« • 



INTRODUCTION. 

Geology is the science which investigates the history of the Earth. Its 
object is to trace the progress of our planet from the earliest beginnings 
of its separate existence, through its various stages of growth, down to 
the present condition of things. It unravels the complicated processes, 
involving vast geographical revolutions, by which each continent and 
country has been built up, tracing out the origin of their materials and 
the manner in which their existing outlines have been determined. It 
likewise follows into detail the varied sculpture of mountain and valley, 
crag and ravine. 

Nor does this science confine itself merely to changes in the inorganic 
world. Geology shows that the present races of plants and animals are 
the descendants of other and very different races that once peopled the 
earth. It teaches that there has been a progress of the inhabitants, as 
well as one of the globe on which they have dwelt ; that each successive 
period in the earth^s history, since the introduction of living things, has 
been marked by characteristic types of the animal and vegetable king- 
doms ; and that, how imperfectly soever they may have been preserved 
or may be deciphered, materials exist for a history of life upon the planet. 
The geographical distribution of existing faunas and floras is often made 
clear and intelligible by geological evidence ; and in a similar way, 
light is thrown upon some of the remoter phases in the history of man 
himself. 

A subject so comprehensive as this must require a wide and varied 
basis of evidence. One of the characteristics of geology is to gather 
evidence from sources which, at first sight, seem far removed from its 
Bcope, and to seek aid from almost every other leading branch of science. 
Thus, in dealing with the earliest conditions of the planet, the geologist 
must fully avail himself of the laboui-s of the astronomer. Whatever is 
ascertainable by telescope, si)ectroscope, or chemical analysis, regarding 
the constitution of other heavenly bodies, has a geological bearing. The 
experiments of the physicist, undertaken to determine conditions of 
matter and of energy, may sometimes be taken as the starting-point of 
^ological investigation. The work of the chemical laboratory forms the 

B 



2 •• INTRODUCTION 



• •- 

• • • 



• • 



founjisiltioh of a vast and increasing mass of geological inquiry. To the 
botanist) the zoologist, even to the unscientific, if observant, traveller by 
hxi^ or sea, the geologist turns for information and assistance. 

'.But while thus culling freely from the dominions of other sciences, 

geology claims, as its peculiar territory, the rocky framework of the 

**'glijbe. In the materials composing that framework, their composition 

•and arrangement, the processes of their formation, the changes which 

. they have individually undergone, and the grand terrestrial revolutions 

. to which they bear witness, lie the main data of geological history. It 

is the task of the geologist to group these elements in such a way that 

they may be made to yield up their evidence as to the march of events 

in the evolution of the planet. He finds that they have in large measure 

arranged themselves in chronological sequence, — the oldest lying at the 

bottom and the newest at the top. Relics of an ancient sea-floor are 

overlain with traces of a vanished land - surface ^ these are in turn 

covered by the deposits of a former lake, above which once more appear 

proofs of the return of the sea. Among these rocky records, too, lie 

the lavas and ashes of long-extinct volcanoes. The ripple left upon 

a sandy beach, the cracks formed by the sun's heat upon the muddy 

bottom of a dried-up pool, the very imprint of the drops of a passing 

rain-shower, have all been accurately preserved, and often bear witness 

to geographical conditions widely different from those that exist where 

such markings are now found. 

But it is mainly by the remains of plants and animals imbedded in 
the rocks that the geologist is guided in unravelling the chronological 
succession of geological changes. He has found that a certain order of 
appearance characterises these organic remains ; that each successive 
group of rocks is marked by its own special types of life ; that these 
types can be recognised, and the rocks in which they occur can be corre- 
lated, even in distant countries, where no other means of comparison are 
available. At one moment, he has to deal with the bones of some large 
mammal scattered through a deposit of superficial gravel, at another 
time, with the minute foraminifers and ostracods of an upraised sea- 
bottom. Corals and crinoids, crowded and crushed into a massive 
limestone on the spot where they lived and died, ferns and terrestrial 
plants matted together into a bed of coal where they originally grew, 
the scattered shells of a submarine sand-bank, the snails and lizards that 
left their mouldering remains within a hollow tree, the insects that have 
been imprisoned within the exuding resin of old forests, the footprints of 
birds and quadrupeds, or the trails of worms left upon former shores — 
these, and innumerable other pieces of evidence, enable the geologist to 
realise in some measure what the vegetable and animal life of successive 
periods has been, and what geographical changes the site of every land 
has undergone. 

It is evident that to deal successfully with these varied materials, a 
considerable acquaintance with diff'erent branches of science is desirable. 
The fuller and more accurate the knowledge which the geologist has of 
kindred branches of inquiry, the more interesting and fruitful will be 



INTRODUCTION 



his own researches. From its very nature, geology demands on the 
part of its votaries, wide sympathy with investigation in almost every 
branch of natural science. Especially necessary is a tolerably large 
acquaintance with the processes now at work in changing the surface 
of the earth, and of at least those forms of plant and animal life whose 
remains are apt to be preserved in geological deposits, or which, in their 
structiu*e and habitat, enable us to realise what their forerunners vere. 

It has often been insisted upon that the Present is the key to the 
Past ; and in a wide sense this assertion is eminently true. Only in 
proportion as we understand the present, where everything is open on all 
sides to the fullest investigation, can we expect to decipher the past, 
where so much is obscure, imperfectly preserved, or not preserved at all. 
A study of the existing economy of nature ought evidently to be the 
foundation of the geologist's training. 

While, however, the present condition of things is thus employed, 
we must obviously be on our guard against the danger of imconsciously 
assuming that the phase of nature's operations which we now witness 
has been the same in all past time ; that geological changes have taken 
place, in former ages, in the manner and on the scale which we behold 
to-day, and that at the present time all the great geological processes, 
which have produced chimges in past eras of the earth's history, are still 
existent and active. Of course, we may assume this uniformity of action, 
and use the assumption as a working hypothesis. But it ought not to 
be allowed a firmer footing, nor on any account be suffered to blind us 
to the obvious truth that the few centuries, wherein man has been 
observing nature, form much too brief an interval by which to measure 
the intensity of geological action in all past time. For aught we can 
tell, the present is an era of quietude and slow change, compared with 
some of the eras that have preceded it. Nor can we be sure that when 
we have explored every geological process now in progress, we have 
exhausted all the causes of change which, even in comparatively recent 
times, have been at work. 

In dealing with the Geological Record, as the accessible solid part of 
the globe is called, we cannot too vividly realise that, at the best, it 
forms but an imperfect chronicle. Geological history cannot be compiled 
from a full and continuous series of documents. Owing to the very 
nature of its origin, the record is necessarily from the first fragmentary, 
and it has been further mutilated and obscured by the revolutions of 
successive ages. Even where the chronicle of events is continuous, it is 
of very unequal value in different places. In one case, for example, it 
may present us with an unbroken succession of deposits, many thousands 
of feet in thickness, from which, however, only a few meagre facts as to 
geological history can be gleaned. In another instance, it brings before 
us, within the compass of a few yards, the evidence of a most varied 
and complicated series of changes in physical geography, as well as an 
abundant and interesting suite of organic remains. These and other 
characteristics of the geological record will become more apparent and in- 
telligible to the student as he proceeds in the study of the science. 



INTRODUCTION 



In the present volume the subject will be distributed under the 
following leading divisions. 

1. The Cosniical Aspects of Gedogy. — It is desirable to realise some 
of the more important relations of the earth to the other members of 
the solar system, of which it forms a part, seeing that geological pheno- 
mena are largely the result of these relations. The form and motions of 
the planet may be briefly touched upon, and attention should be 
directed to the way in which these planetary movements influence 
geological change. The light cast upon the early history of the earth by 
researches into the composition of the sun and stars deserves notice here. 

2. Geognosy — An Inquiry into the Materials of the EartKs Svhstance. — 
This division describes the constituent parts of the earth, its envelopes of 
air and water, its solid crust, and the probable condition of it« interior. 
Especially, it directs attention to the more important minerals of the 
crust, and the chief rocks of which that crust is built up. In this way, 
it lays a foundation of knowledge regarding the nature of the materials 
constituting the mass of the globe, whence we may next proceed to 
investigate the processes by which these materials are produced and 
altered. 

3. Dynamical Geology embraces an investigation of the operations 
which lead to the formation, alteration, and disturbance of rocks, and 
calls in the aid of physical and chemical experiment in elucidation of 
these operations. It considers the nature and operation of the processes 
that have determined the distribution of sea and land, and have moulded 
the forms of the terrestrial ridges and depressions. It further investi- 
gates the geological changes which are in progress over the surface of 
the land and floor of the sea, whether these are due to subterranean 
disturbance, or to the eflbct of operations above ground. Such an 
inquiry necessitates a careful study of the existing economy of nature, 
and forms a fitting introduction to the investigation of the geological 
changes of former periods. This and the previous section, including 
most of what is embraced under Physical Geography and Petrogeny or 
Geogeny, will here be discussed more in detail than is usual in geological 
treatises. 

4. GeotectoniCj or Structural Geology — the Architecture of the Earth, — 
This section of the investigation, applying the results arrived at in the 
previous division, discusses the actual arrangement of the various 
materials composing the crust of the earth. It proves that some have 
been formed in beds or strata, whether by the deposit of sediment on 
the floor of the sea, or by the slow aggregation of organic forms, that 
others have been poured out from subterranean sources in sheets of 
molten rock, or in showers of loose dust, which have been built up into 
mountains and plateaux. It further shows that rocks originally laid 
down in almost horizontal beds have subsequently been crumpled, 
contorted, dislocated, invaded by igneous masses from below, and ren- 
dered sometimes crystalline. It teaches, too, that wherever exposed 
above sea-level, they have been incessantly worn down, and have often 
been depressed, so that older lie buried beneath later accumulations. 



INTRODUCTION 



5. Palceonldogiail Geology. — This branch of the subject deals with the 
organic forms which are found preserved in the rocks of the crust of the 
earth. It includes such questions as the manner in which the remains 
of plants and animals are entombed in sedimentary accumulations, the 
relations between extinct and living types, the laws which appear to have 
governed the distribution of life in time and in space, the nature and use 
of the evidence from organic remains regarding former conditions of 
physical geography, and the relative importance of different genera of 
animals and plants in geological inquiry. 

6. Stratigraphical Geology. — This section might be called Geological 
History, or Historical Geology. It works out the chronological succession 
of the great formations of the earth's crust, and endeavours to trace the 
sequence of events of which they contain the record. More particularly, 
it determines the order of succession of the various plants and animals 
which in past time have peopled the earth, and thus, by ascertaining 
what has been the grand march of life upon the planet, seeks to imravel 
the story of the earth as made kno>vn by the rocks of the crust. 
Further, by comparing the sequence of rocks in one country with that of 
those in another, it furnishes materials for enabling us to picture the 
successive stages in the geographical evolution of the various portions of 
the earth's surface. 

7. Physiographical Geology, starting from the basis of fact laid down by 
stratigraphical geology regarding former geographical changes, embraces 
an inquiry into the history of the present features of the earth's surface 
— continental ridges and ocean basins, plains, valleys, and mountains. It 
investigates the structure of mountains and valleys, compares the moun- 
tains of diflferent countries, and ascertains the relative geological dates of 
their upheaval. It explains the causes on which local differences of 
scenery depend, and shows under what very different circumstances, and 
at what widely separated intervals, the varied contours, even of a single 
country, have been produced. 



BOOK I. 

COSMICAL ASPECTS OF GEOLOGY. 

Before geology had attained to the position of an inductive science, 
it was customary to begin all investigations into the history of the earth 
by propounding or adopting some more or less fanciful hypothesis, in 
explanation of the origin of our planet or of the universe. Such pre- 
liminary notions were looked upon as essential to a right understanding 
of the manner in which the materials of the globe had been put together. 
To the illustrious James Hutton (1785) geologists are indebted, if 
not for originating, at least for strenuously upholding the doctrine that 
it is no part of the province of geology to discuss the origin of thinjgs. 
He taught them that in the materials from which geological evidence is 
to be compiled there can be found " no traces of a beginning, no prospect 
of an end." In England, mainly to the influence of the school which he 
foiuided, and to the subsequent rise of the Geological Society (1807), 
which resolved to collect facts instead of fighting over hypotheses, is due 
the disappearance of the crude and unscientific cosmologies of previous 
centuries. 

But there can now be little doubt that in the reaction against the 
visionary and often grotesque speculations of earlier writers, geologists 
were carried too far in an opposite direction. In allowing themselves to 
believe that geology had nothing to do with questions of cosmogony, 
they gradually grew up in the conviction that such questions could never 
be other than mere speculation, interesting or amusing as a theme for 
the employment of the fancy, but hardly coming >vithin the domain of 
sober and inductive science. Nor would they soon have been awakened 
out of this belief by anything in their own science. It is still true that in 
the data with which they are accustomed to deal, as comprising the sum 
of geological evidence, there can be found no trace of a beginning, though 
there is ample proof of constant, upward progression from some invisible 
starting-point. The oldest rocks which have been discovered on any 
part of the globe have possibly been derived from other rocks older than 
themselves. Geology by itself has not yet revealed, and is little likely 
ever to reveal, a portion of the first solid crust of our globe. If, then, 



8 COSMICAL ASPECTS OF GEOLOGY book i 

geological histx)ry is to be compiled from direct evidence furnished by 
the rocks of the earth, it cannot begin at the beginning of things, but 
must be content to date its first chapter from the earliest period of which 
any record has been preserved among the rocks. 

Nevertheless, though, in its usual restricted sense, geolog}'^ has been, 
and must ever be, unable to reveal the earliest history of our planet, it 
no longer ignores, as mere speculation, what is attempted in this subject 
by its sister sciences. Astronomy, physics and chemistry have in late 
years all contnbuted to cast much light on the earliest stages of the 
earth's existence, previous to the beginning of what is commonly regarded 
as geological history. Whatever extends our knowledge of the former 
conditions of our globe may be legitimately claimed as part of the domain 
of geological inquiry. If Geology, therefore, is to continue worthy of its 
name as the science of the earth, it must take cognisance of these recent 
contributions from other sciences. It can no longer be content to begin 
its annals with the records of the oldest rocks, but must endeavour to 
grope its way through the ages which preceded the formation of any 
rocks. Thanks to the results achieved with the telescope, the spectro- 
scope, and the chemical laboratory, the story of these earliest ages of our 
earth is every year becoming more definite and intelligible. 



1. Relations of the Earth in the Solar System. 

• 

As a prelude to the study of the structure and history of the earth, 
some of the general relations of our planet to the solar system may here 
be noticed. The investigations of recent years, showing the community 
of substance between the difierent members of that system, have revived 
and have given a new form and meaning to the well-known nebular hypo- 
thesis of Kant, Laplace and W. Herschel, which sketched the progress of 
the system from the state of an original nebula to its existing condition 
of a central incandescent sun with surrounding cool planetary bodies. 
According to this hypothesis, the nebula, originally diffused at least 
as far as the furthest member of the system, began to condense towards 
the centre, and in so doing threw oft* or left behind successive rings. 
These, on disruption and further condensation, assumed the form of 
planets, sometimes with a fiu*ther formation of rings, which in the case 
of Saturn remain, though in other planets they have broken up and 
united into satellites. 

Accepting this view, we should expect the matter composing the 
various members of the solar system to be everywhere nearly the same. 
The fact of condensation round centres, however, indicates probable diff*er- 
ences of density throughout the nebula. That the materials composing 
the nebula may have arranged themselves according to their respective 
densities, the lightest occupying the exterior, and the heaviest the 
interior of the mass, is suggested by a comparison of the densities of the 
various planets. These densities are usually estimated as in the follow- 
ing table, that of the earth being taken as the unit : — 



BOOK I 


FLANE'i 


VAR 


Y 01 


UGL 


NS 




y 




Density of the Sun 0*25 






,, Mercury 
5, Venus . 










ri2 

103 






Earth . 










1-00 






,, Mars . 










0-70 






,, Jupiter 
,, Saturn. 










. 0-24 
013 






,, Uranus 










. 0-17 






,, Neptune 










. 0-16 





It is to be observed, however, that " the densities here given are mean 
densities, assuming that the apparent size of the planet or sun is the true 
size, t.d, making no allowance for thousands of miles deep of cloudy 
atmosphere. Hence the numbers for Jupiter, Saturn, and Uranus are 
certainly too small, that for the sun, much too small." ^ Taking the 
figures as they stand, while they do not indicate a strict progression in 
the diminution of density, they state that the planets near the sun 
possess a density about twice as great as that of granite, but that those 
lying towards the outer limits of the system are composed of matter as 
light as cork. Again, in some cases, a similar relation has been observed 
between the densities of the satellites and their primaries. The moon, 
for example, has a density little more than half that of the earth. The 
first satellite of Jupiter is less dense, though the other three are said to 
be more dense than the planet. Further, in the condition of the earth 
itself, a very light gaseous atmosphere forms the outer portion, beneath 
which lies a heavier layer of water, while within these two envelopes the 
materials forming the solid substance of the planet are so arranged that 
the outer layer or crust has only about half the density of the whole 
globe. 

According to the hypothesis now under consideration it is conceived 
that, in the gradual condensation of the original nebula, each successive 
mass left behind represented the density of its parent shell, and consisted 
of progressively heavier matter.- The remoter planets, with their low 
densities and vast absorbing atmospheres, may be supposed to consist of 
metalloids, like the outer parts of the sun's atmosphere, while the interior 
planets are no doubt mainly metallic. The rupture of each planetary 
ring would, it is thought, raise the temperature of the resultant nebulous 
planet to such a height as to allow the vapours to rearrange themselves 
by degrees in successive layers, or rather shells, according to densities. 
And when the planet gave off a satellite, that body might be expected to 
possess the composition and density of the outer layers of its primary.^ 



* Professor Tait, MS. note. 

^ On the origin of Satellites, see the researches of Prof. G. H. Dar>^nn, Phil. Trans. 
(1879) clxx. p. 535. Proc. Boy. Soc. xxx. j). 1. 

' Lockyer in Prestwich's Inaugural Lecture, Oxford, 1875, and in Manchester 
Lectures, Why (he Earth's Chemistry is as it is. Readers interested in the historical 
tlerelopment of geological opinion will find much suggestive matter bearing on the questions 
diflcnssed above, in De U Beche's 'Researches in Theoretical Geology,' 1834,— a work 
notably in advance of its time. 



10 COSMIC AL ASPECTS OF GEOLOGY book i 

For many years, the only evidence available as to the actual com- 
position of other heavenly bodies than our own earth was furnished by 
the meteorites, or fallen stars, which from time to time have entered our 
atmosphere from planetary space, and have descended upon the surface 
of the globe.^ Subjected to chemical analysis, these foreign bodies show 
considerable diversities of composition; but in no case have they yet 
revealed the existence of any element not already recognised among ter- 
restrial materials. They have been classified in three groups: Sideriies, 
composed chiefly of iron ; Siderolites, consisting partly of iron and partly 
of various stony materials ; and Aerolites, formed almost entirely of such 
stony minerals. Twenty-four of our elements have been detected in 
meteorites. Those most commonly found are iron, nickel, phosphorus, 
sulphur, carbon, oxygen, silicon, magnesium, calcium and aluminium. 
Less frequent or occurring in smaller quantities are hydrogen, nitrogen, 
chlorine, lithium, sodium, potassium, titanium, chromium, manganese, 
cobalt, arsenic, antimony, tin and copper. These various elements occur 
for the most part in a state of combination. The iron exists as an alloy 
with nickel, the phosphorus is combined with nickel and iron, the silicon 
is combined with oxygen and various bases. A few of the elements occur 
in a free state. Thus hydrogen and nitrogen are found as occluded gases 
and carbon as graphite, rarely as diamond. Of combinations of elements 
in meteorites some, not yet recognised among terrestrial minerals, comprise 
alloys of iron and nickel and various sulphides and silicates. But others 
have been identified with well-known minerals of the earth's crust, 
including olivine, enstatite and bronzite, diopside and augite, hornblende, 
anorthite and labradorite, magnetite and chromite, &c. There is likewise 
a carbonaceous group of meteorites containing carbon, both amorphous 
and as black diamond, also combined ^vith hydrogen and oxygen, and in 
some cases combustible, with a bituminous smell. Some iron meteorites 
contain a large proportion of occluded hydrogen, nitrogen, or carbonic 
oxide, occasionally as much as six times the volume of the meteorite itsell 

Various theories have been propounded as to the origin or source of 
those bodies which come to our planet from space. But at present we 
possess no satisfactory basis of fact on which to speculate. Whether 
these stones belong to the solar system, or, as seems more probable, reach 
us from remoter space, they prove that some at least of the elements and 
minerals with which we are familiar extend beyond our planet 

But, in recent years, a far more precise and generally available method 

1 On meteorites consult Partsch, * Die Meteoriten,' Vienna, 1843. Rose, Abfiand, IcGnigL 
Akad. Berlin, 1863. Rammelsberg, * Die Chemische Natur der Meteoriten,' 1870-9. Tscher- 
raak, Sitjd), Akad, Wisaen, Vienna (1875), Izzi. ; ' Die Mikroskopische Beschaffenheit der 
Meteoriten,* Stuttgart, 1885. Daubr^e, 'Etudes Synth^tiques de Geologie Exp^rimentale, ' 
1879 ; C(mpt. rend. cvi. (1888), 1671-1682 (compare >4»i«r. Joum, Set. xlii. (1891), p. 413). 
S. Meunier, *Le Ciel Geologique,' 1871; *M^t^orite8,* 1884. Brezina und Cohen, *Die 
Stnictur und Zusammensetzung der Meteoreisen,' Stuttgart, 1886. W. Flight, Oeol. Mag. 
1875, Pop. Set. Rev. new scr. i. p. 390. Proc. Roy. Soc, xxxiii. p. 343. A. W. Wright, 
Amer. Joum. ser. 3, xi. p. 253 ; xii. p. 165. L. Fletcher, * An Introduction to the Study 
of Meteorites,* British Museum Catalogue, 1886. 



BOOK I SPECTROSCOPIC RESEARCH 11 

of research into the composition of the heavenly bodies has been found in 
the application of the spectroscope. By means of this instrument, the 
light emitted from self-luminous bodies can be analysed in such a way as 
to show what elements are present in their intensely hot luminous vapour. 
When the light of the incandescent vapour of a metal is allowed to 
pass through a properly arranged prism, it is seen to give a spectrum 
consisting of transverse bright lines only. This is termed a radiaiion- 
spedrunk Each element appears to have its own characteristic arrange- 
ment of lines, which in general retain the same relative position, intensity 
and colours. Moreover, gases and the vapours of solid bodies are found to 
intercept those rays of light which they themselves emit. The spectrum 
of sodium-vapour, for example, shows among others two bright orange 
lines. If therefore white light, from some hotter light^source, passes 
through the vapour of sodium, these two bright lines become dark lines, 
the light being exactly cut off which would have been given out by the 
sodium itself. This is called an ahsorption-spectram. 

From this method of examination, it has been inferred that many of 
the elements of which our earth is composed must exist in the state of 
incandescent vapour in the atmosphere of the sun. Thirty-two metals 
have been thus identified, including aluminium, barium, manganese, 
lead, calciiun, cobalt, potassium, iron, zinc, copper, nickel, sodium and 
magnesium. These elements, or at least substances which give the same 
groups of lines as the terrestrial elements with which they have been 
identified, do not occur promiscuously diffused throughout the outer mass 
of the sun. According to Mr. Lockyer^s first observations, they appear to 
succeed each other in relation to their respective densities. Thus the 
coronal atmosphere which, as seen in total eclipses, extends to so prodigious 
a distance beyond the disc of the sun, consists mainly of subincandescent 
hydrogen and another element which may be new. Beneath this external 
vaporous envelope lies the chromosphere, where the vapours of incan- 
descent hydrogen, calcium and magnesium can be detected. Further 
inward the spot-zone shows the presence of sodium, titanium, &c. ; while 
stiU lower, a layer (the reversing layer) of intensely hot vapours, lying 
probably next to the inner brilliant photosphere, gives spectroscopic 
evidence of the existence of incandescent iron, manganese, cobalt, nickel, 
copper, and other well-known terrestrial metals.^ 

It is to be observed, however, that in these spectroscopic researches the 

* On spectroscopic research as applied to the sun, see Kirchhoff and Bunsen, 
* Researches on Solar Spectrum,* &c., 1863 ; Angstrom, * Recherches sur le Spectre 
nonnal du Soleil' ; Lockyer, 'Solar Physics/ 1873, and 'Studies in Spectrum Analysis' 
(International Series), 1878 ; Huggins and Miller, Proc. Roy. Soc. xii., PhU. Trans. 1864 ; 
Roscoe's 'Spectrum Analysis,' with authorities there cited. An ingenious theory to 
account for the conservation of solar energy was suggested by the late Sir C. W. 
Siemens {Proc. Roy. Soc. xxxiii. (1881) p. 389). It requires the presence of aqueous 
vapour and carbon compounds in stellar space, which are dissociated and drawn into 
the solar photosphere, where they burst into flame with a large development of heat, 
and then passing into aqueous vapour and carbonic anhydride or oxide, flow to the solar 
equator whence they are projected into space. 



12 COSMICAL ASPECTS OF GEOLOGY book i 

decomposition of the elements by electrical action was not considered. 
The conclusions embodied in the foregoing paragraph have been founded 
on the idea that the lines seen in the spectrum of any element are all due 
to the vibrations of the molecules of that element. But Mr. Lockyer has 
suggested that this view may after all be but a rough approximation 
to the truth ; that it may be more accurate to say, as a result of the 
facts already acquired, that there exist basic elements common to calciimi, 
iron, &c., and to the solar atmosphere, and that the spectrum of each body 
is a summation of the spectra of various molecular complexities which can 
exist at different temperatures, the simplest only being found in the hottest 
part of the sun.^ 

The spectroscope has likewise been successfully applied by Mr. Huggins 
and others to the observation of the fixed stars and nebulae, with the 
result of establishing a similarity of elements between our own system 
and other bodies in sidereal space. In the radiation spectra of nebulae, 
Mr. Huggins finds the hydrogen lines very prominent ; and he conceives 
that they may bo glowing masses of that element. Professor Tait has 
suggested, on the other hand, that they are more probably clouds of 
stones frequently colliding and thus giving off incandescent gases. Sir 
William Thomson (now Lord Kelvin) favom^s this view, which is fiulber 
amply supported by spectroscopic observations. Among the fixed stars, 
absorption-spectra have been recognised, pointing to a structiwe resembling 
that of our sun, viz., an incandescent nucleus which may be solid or liquid 
or of very highly compressed gas, but which gives a continuous spectrum 
and which is surrounded with an atmos[)here of glowing vapour.- Those 
stars which show the simplest spectra are believed to have the highest 
temperature, and in proportion as they cool their materials will become more 
and more differentiated into what we call elements. The most brilliant or 
hottest stars show in their spectra only the lines of gases, as hydrogen. 
Cooler stars, like our sun, give indications of the presence, in addition, of 
the metals — magnesium, sodium, calcium, iron. A still lower temperature 
is marked by the appearance of the other metals, metalloids, and compounds.^ 
The sun would thus be a star .considerably advanced in the process of 
differentiation or association of its atoms. It contains, so far as we know, 
no metalloid except carbon, and possibly oxygen, nor any compound, 
while stars like Sirius show the presence only of hydrogen, with but a 
feeble proportion of metallic vapours ; and on the other hand, the red 
stars indicate by their spectra that their metallic vapours have entered 
into combination, whence it is inferred that their temperature is lower 
than that of our sun. 

More recently, however, another view of the evolution of stars has 
been propounded by Mr. Lockyer. He conceives that all self-luminous 
cosmical bodies are composed either of swarms of meteorites, or of 

' See also the opposite views of Dewar and Liveing, Pro. lioy. Soc. xxx. p. 93, and 
H. W. Vogel, Naiuret xxvii. p. 233. 

- Huggins, Proc. Roy. Soc. 1863-66, and Brit. Assoc. Lecture (Nottingham, 1866) ; 
Huggins and Miller, Phil. Trans. 1864. 

2 Lockyer, Comptes rendm^ Dec. 1873. 



BOOK r FORM AND SIZE OF THE EARTH 13 



masses of vapour produced by collisions of meteorites ; that stars, comets 
and nebulae are only different phases of the same series of changes ; that 
where the temperature of a star is increasing the star consists of a meteor- 
swarm, which by constant collision of its individual meteorites is gradually 
being vapourised by heat ; and that after volatilisation cooling sets in and 
the vapour finally condenses into a globe.^ 

11. Form and Size of the Earth. 

Further confirmation of some of the foregoing views as to the order 
of planetary evolution is furnished by the form of the earth and the 
arrangment of its component materials. 

That the earth is an oblate spheroid, and not a perfectly spherical 
globe, was discovered and demonstrated by Newton. He even calcu- 
lated the amount of ellipticity long before any measurement had con- 
firmed such a conclusion. During the present century numerous arcs of 
the meridian have been measured, chiefly in the northern hemisphere. 
From a series made by different observers between the latitudes of 
Sweden and the Cape of Good Hope, Bessel obtained the following data 
for the dimensions of the earth : — 

Equatorial diameter . . 41,847,192 feet, or 7925*604 miles. 
Polar diameter . . 41,707,314 „ 7899114 ,, 



Amount of polar flattening . 139,768 ,, 26*471 



i> 



The equatorial circumference is thus a little less than 25,000 miles, 
and the difference between the polar and equatorial diameters (neiu'ly 
26^ miles) amounts to about ^^xr^h of the equatorial diameter. ^ More 
recently, however, it has been shown that the oblate spheroid indicated 
by these measurements is not a symmetrical body, the equatorial circum- 
ference being an ellipse instead of a circle. The greater axis of the 
equator lies in long. 8° 15' W. — a meridian passing through Ireland, 
Portugal, and the north-west corner of Africa, and cutting off the north- 
east corner of Asia in the opposite hemisphere.^ 

The polar flattening, established by measurement and calculation as 
that which would necessarily have been assumed by an originally plastic 
globe in obedience to the movement of rotation, has been cited as 
evidence that the earth was once in a plastic condition. Taken in 
connection with the analogies supplied by the sun and other heavenly 
bodies, this inference appeared to be well grounded.* More recently, 

> * The Meteoritic Hypothesis,' 1890. 

' Herschel, 'Astronomy,' p. 139. 

' A. R. Clarke, PhU, Mag. August 1878 ; Encydopctdia BriUinnica^ 9th edit. x. 172. 

^ It was opposed by Mohr (* Geschichte der Erde,' p. 472), who, adopting a suggestion 
long ago made by Playfair, endeavoured to show that the polar flattening can be 
accounted for by greater denudation of the polar tracts, exposed as these have been by the 
heaping up of the oceanic waters towards the equator in consequence of rotation. He dwelt 
chiefly on the effects of glaciers in lowering the land, but as Pfaff has pointed out, the work 
of eroeion is chiefly performed by other atmospheric forces that operate rather towards the 



14 COSMIC AL ASPECTS OF GEOLOGY book i 

however, it has been contended that even in a truly solid body a polar 
flattening might be developed under the influence of rotation.^ 

Though the general spheroidal form of our planet, and probably the 
general distribution of sea and land, are referable to the early effects of 
rotation on a fluid or viscous mass, it is certain that the present details 
of its surface-contours are of comparatively recent date. Speculations 
have been made as to what may have been the earliest character of the 
solid surface, whether it was smooth or rough, and particularly whether 
it was marked by any indication of the existing continental elevations 
and oceanic depressions. So far as we can reason from geological 
evidence, there is no proof of any uniform superficies having ever 
existed. Most probably the first formed crust broke up irregularly, 
and not until after many successive corrugations did the surface 
acquire stability. Some writers have imagined that at first the ocean 
spread over the whole surface of the planet. But of this there is not 
only no evidence, but good reason for believing that it never could have 
taken place. As will be alluded to in a later page, the preponderance 
of water in the southern hemisphere, seems to indicate some excess of 
density in that hemisphere. This excess can hardly have been produced 
by any change since the materials of the interior ceased to be mobile ; it 
must therefore be at least as ancient as the condensation of water on the 
earth's surface. Hence there was probably from the beginning a tendency 
in the ocean to accumulate in the southern rather than in the northern 
hemisphere. 

That land existed from the earliest ages of which we have any record 
in rock-formations, is evident from the obvious fact that these formations 
themselves consist in great measure of materials derived from the waste 
of land. WTien the student, in a later part of this volume, is presented 
with the proofs of the existence of enormous masses of sedimentary 
deposits, even among some of the oldest geological systems, he will 
perceive how important must have been the tracts of land that could 
furnish such piles of detritus. 

The tendency of modem research is to give probability to the 
conception, first outlined by Kant, that not only in our own solar system, 
but throughout the regions of space, there has been a common plan of 
evolution, and that the matter diffused through space in nebulaB, stars, 
and planets is substantially the same as that with which we are familiar. 
Hence the study of the structure and probable history of the sun and 
the other heavenly bodies comes to possess an evident geological interest, 
seeing that it may yet enable us to carry back the story of our planet 
far beyond the domain of ordinary geological evidence, and upon data not 
less trustworthy than those furnished by the rocks of the earth's crust. 

equator than the poles (*Allgemeine Geologie als exacte Wissenschaft/.p. 6). Compare 
Naumaun, NeutaJahrh. 1871, p. 250. Nevertheless, Mohr undoubtedly recalled attention 
to a conceivable cause by which, in spite of polar elevation or equatorial subsidence, the 
external form of the planet might be preserved. 

* See in particular the papers by Mr. C. Chree. PhU. Mag, 1891, pp. 233 
and 342. 



BOOK I THE EARTH'S ROTATION 15 



III. The Movements of the Earth in their Geological Relations. 

We are here concerned with the earth's motions in so far only as they 
materially influence the progress of geological phenomena. 

§ 1. Rotation. — In consequence of its angular momentum at its 
original separation, the earth rotates on its axis. The rate of rotation 
has once been much more rapid than it now is (p. 21). At present a 
complete rotation is performed in about twenty-four hours, and to it is 
due the succession of day and night. So far as observation has yet gone, 
this movement is imiform, though recent calculations of the influence of 
the tides in retarding rotation tend to show that a very slow diminution 
of the angular velocity is in progress. If this be so, the length of the 
day and night will slowly increase until finally the duration of the day 
and that of the year will be equal. The earth will then have reached 
the condition into which the moon has passed relatively to the earth, one 
half being in continual day, the other in perpetual night. 

The linear velocity due to rotation varies in different places, according 
to their position on the surface of the planet At each pole there can be 
no velocity, but from these two points towards the equator there is a 
continually increasing rapidity of motion, till at the equator it is equal to 
a rate of 507 yards in a second. 

To the rotation of the earth are due certain remarkable influences 
upon currents of air circulating either towards the equator or towards the 
poles. Currents which move from polar latitudes travel from parts of 
the earth's surface where the velocity due to rotation is small, to others 
where it is great. Hence they lag behind, and their course is bent more 
and more westward. An air current, quitting the north polar or north 
temperate regions as a north wind, is deflected out of its course, and 
becomes a north-east wind. On the opposite side of the equator, a similar 
current setting out straight for the equator, is changed into a south-east 
wind. Hence, as is well-known, the Trade-winds have their characteristic 
westward deflection. On the other hand, a current setting out north- 
wards or southwards from the equator, passes into regions having a less 
velocity due to rotation than it possesses itself, and hence it travels on in 
advance and appears to be gradually deflected eastward. The aerial 
currents, blowing steadily across the surface of the ocean towards the 
equator, produce oceanic currents which unite to form the westward- 
flowing Equatorial current. 

It has been maintained by Von Baer,^ that a certain deflection is 

* **Ueber ein allgemeines Gesetz in der Gestaltung der Flussbetten." Bull. Acad. St. 
PHerabourgy ii. (1860). See also Ferrel on the motion of fluids and solids relatively to the 
earth's surface, Camh. {Mass.) Math. Monthly^ vols. i. and ii. (1859-60) ; Dulk, X. Deutsch. 
Gtd. Get. xxxi. (1879) p. 224. The River Irtisch is said in flowing northward to have cut 
so much into its- right bank that villages are gradually driven eastwards, Demiansk having 
been shifted about a mile in 240 years (Nature ^ xv. p. 207). But this may be accounteil 
for by local causes. See an excellent paper on this subject with special reference to the 
regime of some rivers in northern Germany, by F. Klockmann, Jahrb. Preuss. Geoi. Landes- 
anst. 1882 ; also E. Dunker, Zeitsch. fur die (jesammten yaturwissenschaften, 1875, p. 463 : 
G. K. Gilbert, Amer, Joum. Set. xxvii. (1884) p. 427. 



16 COSMIC AL ASPECTS OF GEOLOGY book i 

experienced by rivers that flow in a meridional direction, like the Volga 
and Irtisch. Those travelling polewards are asserted to press upon their 
eastern rather than their western banks, while those which run in 
the opposite direction are stated to be thrown more against the western 
than the eastern. When, however, we consider the comparatively small 
volume, slow motion, and continually meandering course of rivers, it may 
reasonably be doubted whether this vera caum can have had much effect 
generally in modifying the form of river-channels. 

j$ 2. Revolution. — Besides turning on its axis, the globe performs a 
movement round the sun, termed revolution. This movement, accom- 
plished in rather more than 365 days, determines for us the length of 
our year, which is, in fact, merely the time required for one complete 
revolution- The path or orbit followed by the earth round the sun is not 
a perfect circle but an ellipse, with the sun in one of the foci, the mean 
distance of the earth from the sun being 92,800,000, the present aphelion 
distance 94,500,000, and the perihelion distance 91,250,000 miles. By 
slow secular variations, the form of the orbit alternately approaches to 
and recedes from that of a circle. At the nearest possible approach 
between the two bodies, owing to change in the ellipticity of the orbit, 
the earth is 14,368,200 miles nearer the sun than when at its greatest 
possible distance. These maxima and minima of distance occur at vast 
intervals of time.^ The last considerable eccentricity took place about 
200,000 years ago, and the previous one more than half a million years 
earlier. Since the amount of heat received by the earth from the sun is 
inversely as the square of the distance, eccentricity may have had in past 
time much effect upon the climate of the earth, as will be pointed out 
further on (§ 8). 

,^ 3. Precession of the Equinoxes. — If the axis of the earth were 
perpendicular to the plane of its orbit, there would be equal day and 
night all the year round. But it is really inclined from that ]X)6ition at 
an angle of 23° 27' 21". Hence our hemisphere is alternately presented 
to and turned away from the sun, and, in this way, brings the familiar 
alternation of the seasons. Again, were the earth a perfect sphere, of 
uniform density throughout, the position of its axis of rotation would 
not be changed by attractions of external bodies. But owing to the 
protuberance along the equatorial regions, the attraction chiefly of the 
moon and sun tends to pull the axis aside, or to make it describe a 
conical movement, like that of the axis of a top, round the vertical. 
Hence each pole points successively to different stars. This movement, 
called the precession of the equinoxes, in combination with another 
smaller movement, due to the attraction of the moon (called nutatioti), 
completes its cycle in 21,000 years, the annual total advance of the 
equinox amounting to 62". At present the winter in the northern hemi- 
sphere coincides with the earth's nearest approach to the sun, or perihelion. 
In 10,500 years hence it will take place when the earth is at the farthest 
part of its orbit from the sun, or in aphelion. This movement may have 

^ See Croll's 'Climate and Time,* chaps, iv., xix. 



BOOK I STABILITY OF EARTH'S AXIS 17 



had great importance in connection with former secular variations in the 
eccentricity of the orbit (§ 8). 

§ 4. Change in the Obliquity of the Ecliptic— The angle at which 
the axis of the earth is inclined to the plane of its orbit does not remain 
strictly constant. It oscillates through long periods of time to the extent 
of about a degree and a half, or perhaps a little more, on either side of 
the mean. According to Dr. Croll,^ this oscillation must have consider- 
ably affected former conditions of climate on the earth, since, when the 
obliquity is at its maximum, the polar regions receive about eight and a 
half days' more of heat than they do at present — that is, about as much 
heat as lat. 76° enjoys at this day. This movement must have augmented 
the geological effects of precession, to which reference has just been made, 
and which are described in § 8. 

§ 5. Stability of the Earth's Axis. — That the axis of the earth's 
rotation has successively shifted, and consequently that the poles have 
wandered to different points on the surface of the globe, has been main- 
tained by geologists as the only possible explanation of certain remarkable 
conditions of climate, which can be proved to have formerly obtained 
within the Arctic Circle. Even as far north as lat. 81° 45', abundant 
remains of a vegetation indicative of a warm climate, and including a bed 
of coal 25 to 30 feet thick, have been found in situ.- It is contended 
that when these plants lived, the ground could not have been permanently 
frozen or covered for most of the year with thick snow. In explanation 
of the difficulty, it has been suggested that the north pole did not occupy 
its present |x>sition, and that the locality where the plants occur lay in 
more southerly latitudes. Without at present entering on the discussion 
of the question whether the geological evidence necessarily requires so 
important a geographical change, let us consider how far a shifting of the 
axis of rotation has been a possible cause of change during that section 
of geological time for which there are records among the stratified rocks. 

From the time of Laplace,^ astronomers have strenuously denied the 
possibility of any sensible change in the position of the axis of rotation. 
It has been urged that, since the planet acquired its present oblate 
spheroidal form, nothing but an utterly incredible amount of deformation 
could overcome the greater centrifugal force of the equatorial protuber- 
ance. It is certain, however, that the axis of rotation does not strictly 
coincide with the principal axis of inertia. Though the angular difference 
between them must always have been small, we can, without having 
recourse to any extramundane influence, recognise two causes which, 
whether or not they may suffice to produce any change in the position 
of the main axis of inertia, undoubtedly tend to do so. In the first 
place, a widespread upheaval or depression of certain unsymmetrically 
arranged portions of the surface to a considerable amount would tend 
to shift that axis. In the second place, an analogous result might arise 
from the denudation of continental masses of land, and the consequent filling 

* CroU, Trans. Oeoh Sac. Glasgow, ii. 177. 'Climate and Time,' chap. xxv. 

- Fielden and Heer, Quart. Journ. Geol. Sac. Nov. 1877. 

* * Mecanique Celeste,' tome v. p. 14. 

C 



18 COSMIC AL ASPECTS OF GEOLOGY book i 

up of sea-basins. Lord Kelvin (Sir William Thomson) freely concedes 
tie physical possibility of such changes. " We may not merely admit,'' he 
says, " but assert as highly probable, that the axis of maximum inertia 
and axis of rotation, always very near one another, may have been in 
ancient times very far from their present geographical position, and may 
have gradually shifted through 10, 20, 30, 40, or more degrees, without 
at any time* any perceptible sudden disturbance of either land or water." ^ 
But though, in the earlier ages of the planet's history, stupendous 
deformations may have occurred, and the axis of rotation may have 
often shifted, it is only the alterations which can possibly have occurred 
during the accumulation of the stratified rocks, that need to be taken 
into account in connection with the evidence of changes of climate 
during geological history. If it can be shown, therefore, that the 
geographical revolutions necessary to shift the axis are incredibly 
stupendous in amount, improbable in their distribution, and not really 
demanded by geological evidence, we may reasonably withhold our 
belief from this alleged cause of the changes of climate during the 
periods of time embraced by geological records. 

It has been estimated by Lord Kelvin "that an elevation of 600 
feet, over a tract of the earth's surface 1000 miles square and 10 miles 
in thickness, would only alter the position of the principal axis by 
one- third of a second, or 34 feet."^ Prof. George Darwin has shown 
that, on the supposition of the earth's complete rigidity, no redistribu- 
tion of matter in new continents could ever shift the pole from its 
primitive position more than 3*^, but that, if its degree of rigidity is 
consistent with a periodical re-adjustment to a new form of equilibrium, 
the pole may have wandered some 10° or 15° from its primitive position, 
, or have made a smaller excursion and returned to near its old place. 
In order, however, that these maximum efiects should be produced, 
it would be necessary that each elevated area should have an area 
of depression corresponding in size and diametrically opposite to it, 
that they should lie on the same complete meridian, and that they 
should both be situated in lat. 45°. With all these coincident favourable 
circumstances, an effective elevation of -^^ of the earth's surface to the 
extent of 10,000 feet would shift the pole 11^'; a similar elevation 
of ^^ would move it 1° 46^'; of ^, 3° 17'; and of i, 8° 4 J'. Mr. 
Darwin admits these to be superior limits to what is possible, and that, 
on the supposition of intumescence or contraction under the regions in 
question, the deflection of the pole might be reduced to a quite 
insignificant amount.^ 

Under the most favourable conditions, therefore, the possible amount 
of deviation of the pole from its first position would appear to have been 
too small to have seriously influenced the climates of the globe within 
geological history. If we grant that these changes were cumulative, and 

* Brit. Assoc. Rep. (1876), Sections, p. 11. 

- Trans. Oeol. Soc. Glasgow^ iv. p. 313. The situation of the supposed area of 
uplieaval on the earth's surface is not stated. 
8 PhU. Trans. Nov. 1876. 



BOOK I STABILITY OF EARTH'S AXIS 19 

that the superior limit of deflection was reached only after a long series 
of concurrent elevations and depressions, we must suppose that no move- 
ments took place elsewhere to counteract the effect of those about lat. 45*^ 
in the two hemispheres. But this is hardly credible. A glance at a 
geographical globe suffices to show how large a mass of land exists now 
both to the north and south of that latitude, especially in the northern 
hemisphere, and that the deepest parts of the ocean are not antipodal to 
the greatest heights of the land. These features of the earth's surface 
are of old standing. There seems, indeed, to be no geological evidence in 
favour of any such geographical changes as could have produced even 
the comparatively small displacement of the axis considered possible by 
Prof. Dsiwin. 

In an ingenious suggestion. Sir John Evans contended that, even 
without any sensible change in the position of the axis of rotation of the 
nucleus of the globe, there might be very considerable changes of 
latitude due to disturbance of the equilibrium of the outer portion 
or shell by the upheaval or removal of masses of land between the 
equator and the poles, and to the consequent sliding of the shell over the 
nucleus until the equilibrium was restored.^ Subsequently he precisely 
formulated his hypothesis as a question to be determined mathematic- 
ally;^ and the solution of the problem was worked out by the 
Rev. J. F. Twisden, who arrived at the conclusion that even the large 
amount of geographical change postulated by Dr. Evans could only 
displace the earth's axis of figure to the extent of less than 10' of 
angle, that a displacement of as much as 10" or 15° could be effected 
only if the heights and depths of the areas elevated and depressed 
exceeded by many times the heights of the highest mountains, that 
under no circumstances could a displacement of 20"" be effected by a 
transfer of matter of less amount than about a sixth part of the whole 
equatorial bulge, and that even this extreme amount would not necessarily 
alter the position of the axis of figure.^ 

Against any h3rpothesis which assumes a thin crust enclosing a 
liquid or viscous interior, weighty and, indeed, insuperable objections 
have been urged. It has been suggested, however, that the almost 
universal traces of present or former volcanic action, the evidence from 
the compressed strata in mountain regions that the crust of the earth 
must have a capacity for slipping towards certain lines, the great 
amount of horizontal compression of strata which can be proved to 
have been accomplished, and the secular changes of climate — notably 
the former warm climate near the north pole — furnish grounds for 
inquiry whether the doctrine of a fluid substratum over a rigid nucleus, 
which has been urged by several able ^mters, would not be compatible 
with mechanical considerations, and whether, under these circumstances, 
changes in latitude would not result from unequal thickening of the 

» Proc Roy. Soc. xv. (1867), p. 46. 2 q j ^,,^^^ ^^ ^^^^^^ (1876), p. 62. 

* g. J, Oeol, Soc. xxxiv. (1878), p. 41. See also E. Hill, Geol. Mag. v. (2nd ser.) 
pp. 262, 479. 0. Fisher, op. cit. pj). 291, 551. 



20 COSMIC A L ASPECTS OF GEOLOGY book i 



crust. ^ This qiiestiou of the internal condition of the globe is dis- 
cussed at p. 47. 

§ 6. Changes of the Earth's Centre of Gravity. — If the centre 
of gravity in our planet, as pointed out by Herschel, ]ye not coincident 
with the centre of figure, but lies somewhat to the south of it, any 
variation in its position will affect the ocean, which of course adjusts 
itself in relation to the earth's centre of gra^dty. How far any redis- 
tribution of the matter within the earth, in such a way as to affect the 
present equilibrium, is now possible, we cannot tell. But certain re- 
volutions at the sui'face may from time to time produce changes of 
this kind. The accumulation of ice which, as will be immediately 
ilescribed (§ 8), is believed to gather round one pole during the 
maximum of eccentricity, will displace the centre of gravity, and, as the 
result of this change, will raise the level of the ocean in the glacial 
hemisphere.^ The late Dr. CroU estimated that, if the present mass of 
ice in the southern hemisphere is taken at 1000 feet thick extending 
down to lat. 60°, the transference of this mass to the northern hemi- 
sphere would raise the level of the sea 80 feet at the north pole. Other 
methods of calculation give difi'erent results. Mr. Heath put the rise at 
128 feet ; Archdeacon Pratt made it more ; while the Rev. O. Fisher 
gave it at 409 feet.^ Subsequently, in returning to this question, Dr. 
CroU remarked " that the removal of two miles of ice from the Antarctic 
continent [and at present the mass of ice there is probably thicker than 
that] would displace the centre of gravity 190 feet, and the formation of 
a mass of ice equal to the one-half of this, on the Arctic regions, would 
carry the centre of gravity 95 feet farther ; gi\ing in all a total displace- 
ment of 285 feet, thus producing a rise of level at the north pole of 285 
feet, and in the latitude of Edinburgh of 234 feet." A very considerable 
additional displacement would arise from the increment of water to the 
mass of the ocean by the melting of the ice. Supposing half of the two 
miles of Antarctic ice to be replaced by an ice-cap of similar extent and 
one mile thick in the northern hemisphere, the other half being melted 
into water and increasing the mass of the ocean, Dr. Croll estimated that 
from this scource an extra rise of 200 feet would take place in the 
general ocean level, so that there would be a rise of 485 feet at the 
north pole, and 434 feet in the latitude of Edinburgh.* An intermittent 
submergence and emergence of the low polar lands might be due to the 
alternate shifting of the centre of gravity. 

To what extent this cause has actually come into opei-ation in j>ast 
time cannot at present be determined. It has been suggested that the 
"raised beaches," shore-lines {straiMinien), or old sea-terraces, so numerous 

» 0. Fisher, Geol. Mag. 1878, p. 552, * Physics of the Earth's Crust/ 1882 ; 2nd Edition 
1889. - Adhemar, 'Revolutions de la Mer,' 1840. 

» CVoll, iu ReacUr for 2nd September, 1865, and Phil, Mag. April, 1866 ; Heath, 
Phil. Mag. April, 1869; Pratt, PhU. Mag. March, 1866: Fisher, Rea4^r, 10th 
February, 1866. 

■• Croll, Geol. Mag. new series, i. (1874), p. 347 ; * Climate and Time,' chaps, xxiii. 
and xxiv. and jw^^fa, p. 286. Consult also Fisher, Phil. Ma^. xxxiv. (October, 1892), p. 337. 



BOOK I INFLUENCE OF SUN AND MOON ON EARTH 21 



at various heights in the north-west of Europe, might be due to the 
transference of the oceanic waters, and not to any subterranean movement, 
as generally believed. Had they been due to such a general cause, they 
ought to have shown evidence of a gradual and uniform decline in elevation 
from north to south, with only such local variations as might be accounted 
for by the influence of masses of high land or other local cause. No such 
feature, however, has been satisfactorily established.^ On the contrary, 
the levels of the terraces vary within comparatively short distances. 
Though numerous on both sides of Scotland, they disappear further north 
among the Orkney and Shetland islands, although these localities were 
admirably adapted for their formation and preservation.*^ The conclusion 
may be drawn that the " raised beaches " cannot be adduced as evidence 
of changes of the earth's centre of gravity, but are due to local and 
irregularly acting causes. (See Book III. Part I. Section iii. § 1, where 
this subject is more fully discussed.) 

§ 7. Results of .the Attractive Influence of Sun and Moon on the 
Geologrical Condition of the Earth. — Many speculations have been oflered 
to account for supposed former greater intensity of geological activity on 
the surface of the globe. Two causes for such greater intensity may be 
adduced. In the first place, if the earth has cooled down from an 
original molten condition, it has lost, in cooling, a vast amount of 
potential geological energy. It does not necessarily follow, however, 
that the geological phenomena resulting from internal temperature have, 
during the time recorded in the accessible part of the earth's crust, been 
steadily decreasing in magnitude. We might, on the contrary, contend 
that the increased resistance of a thickening cooled crust may rather 
have hitherto intensified the manifestations of subterranean activity, by 
augmenting the resistance to be overcome. In the second place, the 
earth may have been once more powerfully affected by external causes, 
such as the greater heat of the sun, and the greater proximity of the 
moon. That the formerly larger amount of solar heat received by the 
surface of our planet must have produced warmer climates and more 
rapid evaporation, with gi*eater rainfall and the important chain of 
geological changes which such an increase would introduce, appears in 
every way probable, though the geologist has not yet been able to observe 
any indisjmtable indication of such a former intensity of superficial 
changes. 

Prof. Darwin, in investigating the bodily tides of viscous spheroids, 
has brought forward some remarkable results bearing on the question 
of the possibility that geological operations, both internal and superficial, 
may have been once greatly more gigantic and rapid than they are 
now.^ He assumes the earth to be a homogeneous spheroid and to have 
possessed a certain small \iscosity,* and he calculates the internal tidal 

* The student ought, however, to consult Prof. Suess' Antlitz tier Erde for the arguments 
in favour of an opposite opinion. 

- XcUure, xvi. (1877), p. 415. •' Phil, Trans. 1879. parts i. and ii. 

"* The degree of viscosity assumed is such tliat **thirteeu and a half tons to the 
square inch acting for twenty -four hours on a slab an inch thick displaces the upper 



22 COSMICAL ASPECTS OF GEOLOGY book i 

friction in such a mass exposed to the attraction of moon and sun, and 
the consequences which these bodily tides have produced. He finds that 
the length of our day and month have greatly increased, that the 
moon's distance has likewise augmented, that the obliquity of the 
ecliptic has diminished, that a large amount of hypogene heat has been 
generated by the internal tidal friction, and that these changes may all 
have transpired within comparatively so short a period (57,000,000 years) 
as to place them quite probably within the limits of ordinary geological 
history. According to his estimate, 46,300,000 years ago the length of 
the sidereal day was fifteen and a half hours, the moon's distance in mean 
radii of the earth was 46*8 as compared with 60*4 at the present time. 
But 56,810,000 years back, the length of a day was only 6|^ hours, or 
less than a quarter of its present value, the moon's distance was only 
nine earth's ladii, while the lunar month lasted not more than about a 
day and a half (1*58), or ^j of its present duration. He arrives at the 
deduction that the energy lost by internal tidal friction in the earth's 
mass is converted into heat at such a rate that the amount lost during 
57,000,000 years, if it were all applied at once, and if the earth had the 
specific heat of iron, would raise the temperature of the whole planet's 
mass 1,760^ Fahrenheit, but that the distribution of this heat-generation 
has been such as not to interfere with the normal augmentation of 
temperature downward due to secular cooling, and the conclusion drawn 
therefrom by Sir William Thomson. Mr. Darwin further concludes from 
his hypothesis that the ellipticity of the earth's figure having been 
continually diminishing, " the polar regions must have been ever rising 
and the equatorial ones falling, though as the ocean followed these 
changes, they might quite well have left no geological traces. The tides 
must have been very much more frequent and larger, and accordingly 
the rate of oceanic denudation much accelerated. The more rapid 
alternation of day and night ^ would probably lead to more sudden and 
violent storms, and the increased rotation of the earth would augment 
the violence of the trade-winds, which in their turn would affect oceanic 
currents." ^ As above stated, no facts yet revealed by the geological 
record compel the admission of more violent superficial action in former 
times than now. But though the facts do not of themselves lead to such 
an admission, it is proper to enquire whether any of them are hostile to 
it. It will be shoAvn in Book VI. that even as far back as early Palaeozoic 
times, that is, as far into the past as the history of organised life can be 
traced, sedimentation took place very much as it does now. Sheets of 
fine mud and silt were pitted with rain drops, ribbed with ripple-marks, 
and furrowed by crawling worms, exactly as they now are on the shores 
of any modern estuary. These surfaces were quietly buried under 

surface relatively to the lower through one -tenth of an inch. It is obvious," says Mr. 
Darwin, "that such a substance as this would be called a solid in ordinarj' parlance, 
and in the tidal problem this must be regarded as a very small viscosity." Op, cit. 
p. 531. 

^ According to his calculation, the year 57,000,000 of years ago contained 1300 days 
instead of 365. ^ Op, cit. p. 532. 



BOOK I CLIMATE IN ITS GEOLOGICAL RELATIONS 23 



succeeding sediment of a similar kind, and this for hundreds and 
thousands of feet. Nothing indicates violence ; all the evidence favours 
tranquil deposit^ K, therefore, Mr. Darwin's hypothesis be accepted, 
we must conclude either that it does not necessarily involve such violent 
superficial operations as he supposes, or that eveA the oldest sedimentary 
formations do not date back to a time when the influence of increased 
rotation could make itself evident in sedimentation, that is to say, on 
Mr. Darwin's h3rpothesis, the most ancient fossiliferous rocks cannot be 
as much as 57,000,000 years old. * 

§ 8. Climate in its Geolosrical Relations. — In subsequent parts of 
this volume data will be given from which we learn that the climates of 
the earth have formerly been considerably different from those which at 
present prevail. A consideration of the history of the solar system 
would of itself suggest the inference that, on the whole, the climates of 
early geological periods must have been warmer. The sun's heat was 
greater, probably the amount of it received by the earth was likewise 
greater, while there would be for some time a sensible influence of the 
planet's own internal heat upon the general temperature of the whole 
globe.^ Although arguments based upon the probable climatal neces- 
sities of extinct species and genera of plants and animals must be used 
with extreme caution, it may be asserted with some confidence that from 
the vast areas over which Palaeozoic moUusks have been traced, alike 
in the eastern and the western hemispheres, the climates of the globe 
in Palaeozoic time were probably more uniform than they now are. 
There appears to have been a gradual lowering of the general tempera- 
ture during past geological time, accompanied by a tendency towards 
greater extremes of climate. But there are proofs also that at longer 
or shorter intervals cold cycles have intervened. The Glacial Period, 
for example, preceded our own time, and in successive geological forma- 
tions indications, of more or less value, have been found that suggest if 
they do not prove a former prevalence of ice in what are now temperate 
regions.' 

* Sir R Ball {Nature, xxv. 1881, pp. 79, 103), starting from Professor Darwin's data, 
pushed his conclusions to such an extreme as to call in the agency of tides more than 600 
feet high in early geological times. In repudiating this application of his results, Mr. 
Darwin {Xaturt, xxv. p. 213) employs the argument I have here used fh)m the absence of 
any evidence of such tidal action in the geological formations, and f^om the indication, on 
the contrary, of tranquil deposit 

' Lord Kelvin (Sir William Thomson) believes that the hypothesis that terrestrial tempera- 
ture was formerly higher by reason of a hotter sun " is rendered almost infinitely probable 
by independent physical evidence and mathematical calculation." (Trans. Geol. Soc. 
Olasffowj V. p. 238.) Profes»or Tait, however, has suggested, that the former greater 
heat of the sun may have raised such va.st clouds of absorbing vapour round that 
Inminary as to prevent the effective amount of radiation of heat to the earth's surface 
from being greater than at present ; while on the other hand, a similar supposition may be 
made with reference to the greater amount of vajwur which increased solar radiation would 
raise to be condensed in the earth's atmosphere. * Recent Advances in Physical Science, ' 
1876, p. 174. 

» Consult a suggestive paper by the late Dr. M. Neumayr, Nature, xlii. (1890), p. 148. 



24 COSMICAL ASPECTS OF GEOLOGY book i 



Various theories have been proposed in explanation of such alternations 
of climate. Some of these have appealed to a change in the position of 
the earth's axis relatively to the mass of the planet (an7e, § 5). Others 
have been based on the notion that the earth may have passed through 
hot and cold regions of space. Others, again, have called in the effects 
of terrestrial changes, such as the distribution of land and sea, on the 
assumption that elevation of land about the poles must cool the temperature 
of the globe, while elevation round the equator would raise it.^ But the 
changes of temperature appear to have affected the whole of the earth's 
surface, while there is not only no proof of any such enormous vicissitudes 
in physical geography as would be required, but good grounds for 
believing that the present terrestrial and oceanic areas have remained, 
on the whole, on the same sites from very early geological time. More- 
over, as evidence has accumulated in favour of periodic alternations of 
climate, the conviction has been strengthened that no mere local changes 
could have sufficed, but that secular variations in climate must be assigned 
to some general and probably recurring cause. 

By degrees, geologists accustomed themselves to the belief that the 
cold of the Glacial Period was not due to mere terrestrial changes, but 
was to be explained somehow as the result of cosmical causes. Of various 
suggestions as to the probable nature and operation of these causes, one 
deserves careful consideration — change in the eccentricity of the earth's 
orbit. Sir John Herschel *^ pointed out many years ago that the direct 
effect of a high condition of eccentricity is to produce an unusually cold 
Avinter, followed by a correspondingly hot summer, in the hemisphere 
whose winter occurs in aphelion, while an equable condition of climate 
at the same time prevails on the opposite hemisphere. But both hemi- 
spheres must receive precisely the same amount of solar heat, because 
the deficiency of heat, resulting from the sun's greater distance during 
one part of the year, is exactly com}>ensated by the greater length of that 
season. Sir John Herschel even considered that the direct effects of 
eccentricity must thus l>e nearly neutralised.^ As a like verdict was 
afterwards given by Arago, Humboldt, and others, geologists were satisfied 
that no importiint change of climate could be attributed to change of 
eccentricity. 

The late Dr. James CroU, as far back as the year 1864, made an im- 
portant suggestion in this matter, and subsequently worked out an 
elaborate development of the whole subject of the physical causes on 
which climate depends."* He was good enough to draw up the following 
abstract of them for former editions of the present work. 

"Assuming tlie mean distance of the auu to \)e 92,400,000 miles, then when the 
eccentricity is at its 8Uj>erior limit, '07776, the distance of the sun from the earth, 

' In Lyell's * Principles of Geology,' this doctrine of the influence of geographical changes 
is maintained. 

* Tnins. (wed, Soc. vol, iii. p. 293 (2ud series). 

^ ' Cabinet Cyclopaedia,* sec. 315; 'Outlines of Astronomy,' sec. 368. 

* Phil. Mag. xxviii. (1864), p. 121. His researches will be found in detail in his volume 
'Climate and Time,' 1875, and his later work 'Discussions on CMinmte and Cosmology.' 



BOOK I EFFECTS OF ECGENTRIGITY OF EARTHS ORBIT 



25 



when the latter is in. the aphelion of its orbit, is no less than 99,584,100 miles, and 
when in the perihelion it is only 85,215,900 miles. The cai-th is, therefore, 14,368,200 
miles farther from the snn]in the former than in the latter position. ^^The direct heat 
of the sun being inversely as the square of the distance, it follows that the amount of 
heat received by the earth in these two positions will be as 19 to 26. The present 
eccentricity being -0168, the earth's distance during our northern winter is 90,847,680 
miles. Suppose now that, irom the precession of the equinoxes, winter in our northern 
hemisphere should happen when the earth is in the aphelion of its orbit, at the time 
that Uie orbit is at its greatest eccentricity ; the earth would then be 8,736,420 miles 
farther from the sun in winter than it is at present. The direct heat of the sun would 
therefore, during winter, be one-fifth less and during summer one-fifth greater than now. 



HP. 





•-P. 



H.P. 



y. Winter SoUti4x in Aphelion. N. Winter Solslke in Perihelion, 

Fig. 1.— Eccentricity of the Eftrtirs Orbit in Relation to Climate. 

This enormous difference would necessarily affect tlie climate to a very great extent. 
Were the wintera under these circumstances to occur when the earth was in the perihelion 
of its orbit, the earth would then be 14,368,200 miles nearer the sun in winter than in 
summer. In this case the difference between winter and sunmier in our latitudes would 
be almost annihilated. But as the winters in the one liemi8j)here correspond with the 
summers in the other, it follows that while the one hemisphere would be enduring the 
greatest extremes of summer heat and winter cold, the otlier would be enjoying perpetual 
summer. 

** It is quite tnie that, whatever may be the eccentricity of the earth's orbit, the two 
hemispheres must receive equal quantities of heat [)er annum ; for proximity to the sun 
is exactly compensated by the effect of swifter motion. The total amount of heat 
received from the sun between the two equinoxes is, therefore, the same in both halves 
of the year, whatever the eccentricity of the earth's orbit may he. For example, whatever 
extra heat the southern hemisphere may at present receive i)cr day from the sun during 
its summer months, owing to greater j)roximity to the sun, Ls exactly com})ensated by a 
corresponding loss arising from the shortness of the season ; and, on the other hand, 
whatever deficiency of heat we in the northern hemisphere may at jiresent have i>er day 
during our summer half-year, in consequence of the earth's distance from the sun, is also 
exactly compensated by a corres|)onding length of season. 

•* It is well known, however, that those simple changes in the summer and winter 



26 COSMICAL ASPECTS OF GEOLOGY book i 

distances would not alone produce a glacial epoch, and that physicists, confining their 
attention to the purely astronomical effects, were perfectly correct in affirming that no 
increase of eccentricity of the earth's orbit could account for that epoch. But the im- 
portant fact was overlooked that, although the glacial epoch could not result directly 
from an increase of eccentricity, it might nevertheless do so indirectly from physical 
agents that were brought into operation as a result of an increase of eccentricity. The 
following is an outline of what these physical agents were, how they were brought into 
operation, and tlie way in which they may have led to the glacial epoch. 

" With the eccentricity at its superior limit and the winter occurring in the aphelion, 
the earth would, as we have seen, be 8,736,420 miles farther from the sun during that 
season than at present. The reduction in the amount of heat received from the sun 
owing to his increased distance, would lower the midwinter temperature to an enormous 
extent. In temperate regions the greater portion of the moisture of the air is at present 
precipitated in the form of rain, and the very small portion which falls as snow disappears 
in the course of a few weeks at most. But in the circumstances under consideration, 
the mean winter-temperature would be lowered so much below the freezing-point that 
what now falls as rain during that season, w^ould then fall as snow. This is not all ; the 
winters would then not only be cooler than now, but they would also be much longer. 
At present the wiutei*s are nearly eight days shorter than the summers ; but with 
the eccentricity at its superior limit and the winter solstice in aphelion, the length of 
the winters would exceed that of the summers by no fewer than thirty-six days. The 
lowering of the temperature and the lengthening of the winter would both tend to the 
same effect, viz., to increase the amount of snow accumulated during the winter ; for, 
other things being equal, the longer the snow-accumulating period the greater the 
accumulation. It may be remarked, however, that the absolute quantity of heat received 
during winter is not affected by the decrease in the sun's heat, for the additional length 
of the season compensated for this decrease.^ As regards the absolute amount of heat 
received, increase of the sun's distance and lengthening of the "winter are compensatory, 
but not so in regard to the amount of snow accumulated. The consequence of this state 
of things would be that, at the commencement of the short summer, the ground would 
be covered with the winter's accumulation of snow. Again, the presence of so much 
snow would lower the summer tem^ierature, and prevent to a great extent the melting of 
the snow. 

"There arc three separate ways whereby accimuilated masses of snow and ice tend 
to lower the summer temperature, viz. : — 

** First, By means of direct radiation. No matter what the intensity of the sun's 
rays may be, the temjierature of snow and ice can never rise above 32*. Hence, the 
presence of snow and ice tends by direct radiation to lower the temperature of all 
surrounding bodies to 32°. In Greenland, a country covered with snow and ice, the 
pitch has been se«n to melt on the side of a ship exposed to the direct rays of the sun, 
while at the same time, the surrounding air was far l)elow the freezing-point ; a thermo- 
meter exposed to the dii*ect radiation of the sun has been observed to stand above 100*, 
while the air surrounding the instrument was actually 12* below the freezing-point. A 
similar experience has been recorded by travellers on the snow -fields of the AIjjs. These 
results, surjnising as they no doubt apjiear, are what we ought to exi)ect under the 
circumstances. Perfectly dry air seems to be nearly incapable of absorbing radiant heat. 
The entire radiation jyasses through it almost without any sensible absorption. Conse- 
quently the pitch on the side of the ship may be melted, or the bulb of the thennometer 
raised to a high temperature by the direct rays of the sun, while the suiTounding air 



' When the eccentricity is at it« superior limit, tlie absolute quantity of heat received by 
the earth during the year is, however, about one three-hundredth part greater than at present. 
But this does not affect the question at issue. 



BOOK I CAUSE OF GLACIAL CLIMATES 27 

reniAins intensely cold. The air is cooled by conUut with the snow-covered ground, but 
is not heated by the radiation from the sun. 

'* When the air is charged with aqueous vapour, a similar cooling effect also takes 
place, but in a slightly different way. Air charged with aqueous vapour is a good 
absorber of radiant heat, but it can only absorb those rays which agree with it in period. 
It 80 happens that rays from snow and ice are, of all others, those which it absorbs best. 
The humid air will absorb the total radiation from the snow and ice, but it will allow 
the greater part of, if not nearly all, the sun's rays to pass unabsorbed. But during the 
day, when tiie sun is shining, the radiation from the snow and ice to the air is negative ; 
that is, the snow and ice cool the air by radiation. The result is, the air is cooled by 
radiation fix>m the snow and ice (or rather, we should say, to the snow and ice) more 
rapidly than it is heated by the sun ; and as a consequence, in a country like Greenland, 
covered with an icy mantle, the temperature of the air, even during summer, seldom 
rises above the freezing-point. Snow is a good reflector, but as simple reflection does not 
change the chsffacter of the rays, they would not be absorbed by the air, but would pass 
into stellar space. Were it not for the ice, the summers of North Greenland, owing to 
the continuance of the sim above the horizon, would be as warm as those of Ehigland ; 
but instead of this, the Greenland summers are colder than our winters. Cover India 
with an ice sheet, and its summers would be colder than those of England. 

** Second f Another cause of the cooling effect is that the ra^-s which fall on snow and 
ice are to a great extent reflected back into space. But those that are not reflected, but 
absorbed, do not raise the temperature, for they disappear in the mechanical work of 
melting the ice. For whatsoever may be the intensity of the sun's heat, the surface of 
the ground will be kept at 32° so long as the snow and ice remain unmelted. 

Third, Snow and ice lower the temperature by chilling the air and condensing the 
vapour into thick fogs. The great strength of the sun's rays during summer, due to his 
nearness at that season, would, in the first place, tend to produce an increased amount of 
evaporation. But the presence of snow-clad mountains and an ic}' sea would chill the atmo- 
sphere and condense the vapour into thick fogs. The thick fogs and cloudy sk}' would 
effectually prevent the sun's rays from reaching the earth, and the snow, in consequence, 
would remain unmelted during the entire summer. In fact, we have this very condition 
of things exemplified in some of the islands of the Southern Ocean at the present da}'. 
Sandwich Land, which is in the same parallel of latitude as the north of Scotland, is 
covered with ice and snow the entire summer ; and in the island of South Georgia, which 
is in the same parallel as the centre of England, the perjwtual snow descends to the ver}- 
sea-beach. Captain Sir James Ross found the perpetual snow at the sea-level at Admir- 
alty Inlet, South Shetland, in lat 64** ; and while near this place the thermometer in 
the very middle of summer fell at night to 23" F. The reduction of the sun's heat and 
lengthening of the winter, which would take place when the eccentricit}' is near to its 
superior limit and the winter in aphelion, would in this countiy produce a state of things 
perhaps as bad as, if not worse than, that wliich at present exists in South Georgia and 
South Shetland. 

** The cause which above all others must tend to produce great changes of climate, is 
the deflection of great ocean currents. A high condition of eccentricity tends, we have 
seen, to produce an accimiulation of snow and ice on the hemisphere whose wintei-s occur 
in aphelion. The accumulation of snow, in turn, tends to lower the simimer temperature. 
cut off the sun's rays, and retard the melting of the snow. In short, it tends to produce. 
on that hemisphere, a state of glaciation. Exactly opjiosite effects take place on the 
other hemisphere, which has its winter in [perihelion. There the shortness of tlic winters, 
combined with the high temperature arising from the nearness of the sun, tends to 
prevent the accumulation of snow. Tlie general result is that the one hemisphere Is 
cooled and the other heated. This state of things now brings into play the agencies which 
lead to the deflection of the Gulf-stream and other great ocean euiTents. 

** Owing to the great difference between the tem|)erature of the equator and the poles, 



28 COSMIC AL ASPECTS OF GEOLOGY book i 



there is a constant flow of air from the yioles to the equator. It is to this that the trade- 
winds owe their existence. Now, as the strength of these winds will, as a general rule, 
depend upon the difference of tenijjerature that may exist between the equator and 
higher latitudes, it follows that the trades on the cold hemisphere will be stronger than 
those on the warm. When the jwlar and temyierate regions of the one hemisphere are 
covered to a large extent with snow and ice, the air, as we have just seen, is kept 
almost at the freezing-{>oint during both summer and winter. The trades on that hemi- 
sphere will, of necessity, he exceedingly ywwerful ; while on the other hemisphere, where 
there is comiMtrativcly little snow or ice, and the air is warm, the trades will consequently 
be weak. Supjwse now the northern hemisphere to be the cold one. Tlie north-east 
trade- winds of this hemisphere will far exceed in strength the south-east trade-winds of 
the southern hemisphere. The otiedian line between the tra<les will consetjuently lie to 
a very Gonaidera1>le distance to the south of the equator. We have a good example of 
this at the present day. The difference of temperature lietween the two hemispheres at 
present is but trifling to what it would be in the ca.se under consideration ; yet we find 
that the south-east trades of the Atlantic blow with gi*eater force than the north-east 
trades, sometimes extending to 10" or Ifi** X. lat,, whereas the north-east trades seldom 
l)low south of the equator. The effect of the northern trades blowing across the equator 
to a grcAt distance will be to imjiel the warm water of the tropics over into the Southern 
Ocean. But this is not all ; not only would the median line of the trades be shifted 
southwards, but the great equatorial currents of the globe would also be shifted southwards. 

" Let us now consider how this would affect the Gulf-stream. The South American 
continent is shaped somewhat in the fomi of a triangle, with one of its angular comers, 
called Cape St, Roque, ]>ointing eastwards. Tlie equatorial current of the Atlantic 
impinges against this corner ; but as the greater jwrtion of the current lies a little to the 
north of the comer, it flows westward into the Gulf of Mexico and fonns the Gulf-stream. 
A considerable portion of the water, however, strikes the land to the south of the cajie, 
and is deflected along the shorc of Brazil into the Southern Ocean, forming what is 
known as the Brazilian current Now, it is obvious that the shifting of the equatorial 
current of the Atlantic only a few degi-ees to the south of its i>resent position — a thing 
which would certainly take place under the conditions which we have heen detailing — 
would turn the entire current into the Brazilian l^ranch, and instead of flowing chiefly 
into the Gulf of Mexico, as at present, it would all flow into the Southern Ocean, and 
the Gulf-stream would consequently be stopped. The stopjwge of the Gulf-stream, 
combined with all those causes which we have just been considering, would place Eurojie 
under a glacial condition, while at the same time the temjierature of the Southern Ocean 
would, in consequence of the enormous ([uantity of warm water received, have its 
temperatin*e (alread}^ high from other causes) raised enoimously. And what holds true 
in regard to the cun'ents of the Atlantic holds also tme, though j)erha})S not to the same 
extent, of the cun-ents of the Pacific. 

" If the breadth of the Gulf-stream be taken at 50 miles, its depth at 1000 feet, its 
mean velocity at 2 statute miles an hour, the temperature of the water when it leaves 
the Gulf at 65**, and the return cun*ent at 40° F.,^ then, the quantity of heat conveyed 
into the Atlantic by this stream is equal to one-fourth of all the heat received from the 
sun by that ocean from the Tropic of Cancer to the Arctic Circle.- From principles 



^ Sir Wyville Thomson states tliat in May, 1873, the CfiuUenger expedition found 
the Gulf-stream, at the ])oint where it was crossed, to be about sixty miles in width, 
100 fathoms deep, and flowing at the rate of three knots per hour. This makes the 
volume of the stream one-fifth greater than the above estimate. 

- The quantity of heat conveyed by the Gulf-stream for distribution is equal to 
77,479,650,000,000,000,000 foot-pounds j^er day. The quantity received from the sun 
by the North Atlantic is 310,923,000,000,000,000,000 foot - i)0unds. 'Climate and 
Thne,' chap. ii. 



30 COSMICAL ASPECTS OF GEOLOGY book i 

to the deduction from this alternation or periodicity that they have 
probably been due to some general or cosmical cause. Dr. CroU 
ingeniously showed that every long cold period arising in each hemisphere 
from the circumstances sketched in the preceding pages, must have been 
interrupted by several shorter warm periods. 

"When the one hemisphere," he says, "is under glaciation, the other is enjoying a 
warm and equable climate. But, owing to the precession of the equinoxes, the condition 
of things on the two hemispheres must be reversed every 10,000 years or so. When the 
solstice passes the aphelion, a contrary process commences ; the snow and ic^ gradually 
begin to diminish on the cold hemisphere and to make their appearance on the other 
hemisphere. The glaciated hemisphere turns by degrees warmer, and the warm hemi- 
sphere colder, and this continues to go on for a period of ten or twelve thousand years, 
until the winter solstice reaches the perihelion. By this time the conditions of the two 
hemispheres have been reversed ; the formerly glaciated hemisphere has now become the 
warm one, and the warm hemisphere the glaciated. The transference of the ice from 
the one hemisphere to the other continues as long as the eccentricity remains at a high 
value. It is probable that, during the warm inter-glacial periods, Greenland and the 
Arctic regions would be comparatively free from snow and ice, and enjoying a temperate 
and equable climate." 



BOOK 11. 

GEOGNOSY. 

AN INVESTIGATION OF THE MATERIALS OF THE EARTH'S 

SUBSTANCE. 

Part I. — A General Description of the Parts of the Earth. 

A discussion of the geological changes which our planet has undergone, 
ought to be preceded by a study of the materials of which the planet 
consists. This latter branch of inquiry is termed Geognosy. 

Viewed in a broad way, the earth may be considered as consisting 
of (1) two envelopes, — an outer one of gas (atmosphere), completely sur- 
rounding the planet, and an inner one of water (hydrosphere), covering 
about three-fourths of the globe; and (2) a globe (lithosphere), cool and 
solid on its surface, but possessing a high internal temperature. 

I. — The Envelopes — Atmosphere and Hydrospliere. 

It is certain that the present gaseous and liquid envelopes of the 
planet form only a portion of the original mass of gas and water with 
which the globe was invested. Fully a half of the outer shell or crust of 
the earth consists of oxygen, which, there can be no doubt, once existed 
in the atmosphere.. The extent, likewise, to which water has been 
abstracted by minerals is almost incredible. It has been estimated that 
already one-third of the whole mass of the ocean has been thus absorbed. 
Eventually the condition of the planet will probably resemble that of 
the moon — a globe without air, or water, or life of any kind. 

1. The Atmosphere. — The gaseous envelope to which the name of 
atmosphere is given, extends to a distance of perhaps 500 or 600 miles 
from the earth's surface, possibly in a state of extreme tenuity to a 
still greater height But its thickness must necessarily vary with lati- 
tude and changes in atmospheric pressure. The layer of air lying over 
the poles is not so deep as that which surrounds the equator. 

Many speculations have been made regarding the chemical composition 



32 GEOGXOSy book u 



of the 'atmosphere chiriDg former geological periods. There can indeed 
})e no doubt that it must originally have differed very greatly from its 
present condition. Besides the abstraction of the oxygen which now 
forms fully a half of the outer crust of the earth, the vast beds of coal 
found all over the world, in geological formations of many different ages, 
doubtless represent so much carbon-dioxide (carbonic acid) once present 
in the air. Acconling to Sterry Hunt, the amount of carbonic acid 
absorbed in the process of rock-decay, and now represented in the form 
of carbonates in the earth*s crust, probably equals two hundred times the 
present volume of the entire atmosphere.^ The chlorides in the sea, 
likewise, were probably carried down out of the atmosphere in the 
primitive condensation of aqueous vapour. It has often been stated 
that, during the Carboniferous period, the atmosphere must have been 
warmer and i^ith more aqueous vapour and carbon-dioxide in its com- 
position than at the present day, to admit of so luxuriant a flora as that 
from which the coal-seams were formed. There seems, however, to be at 
present no method of arriving at any certainty on this subject 

As now existing, the atmosphere is considered to be normally a 
mechanical mixtiu-e of nearly 4 vohunes of nitrogen and 1 of oxygen 
(N79'4, 020*6), i»ith minute proportions of cai*bon- dioxide and ni-ater- 
vapour and still smaller quantities of ammonia and the powerfiU 
oxidising agent, ozone. These quantities are liable to some variation 
according to locality. The mean proportion of carbon -dioxide is about 3 '5 
parts in every 10,000 of air. In the air of streets and houses the pro- 
portion of oxygen diminishes, while that of carbon -dioxide increases. 
According to the researches of Angus Smith, very pure air should 
contain not less than 20*99 per cent, of oxygen, i^-ith 0*030 of carbon- 
dioxide ; but he found impure air in Manchester to have onlj' 20*21 <rf 
oxygen, while the j>roportion of carbon-dioxide in that city during fog 
was ascertained to rise sometimes to 0*0679, and in the pit of the theatre 
to the very large amount of 0*2734. As plants absorb carbon -dioxide 
during the day and give it off at night, the quantity of this gas in the 
atmosphere oscillates between a maximiun at night and a minimum 
during the day. During the part of the year when vegetation is active, 
it is believed that there is at least 10 per cent, more carbonic acid in 
the air of the oj)en country at night than in the day.^ Small as the 
normal percentage of this gas in the air may seem, -yet the total amount 
of it in the whole atmosphere probably exceeds what would be disengaged 
if all the vegetable and animal matter on the earth's surface were burnt. 

The other sul>stances in the air are gases, vapours, and solid particles. 
Of these by much the most important is the vapour of water, which is 
always present, but in very variable amount according to temi)erature.* 

* Brit. Asgoc. Hep. 1878, Sects, p. 544. 

- Prof. G. F. Arnistrong. Proc. Hot/. .Swc. xxx. (1880), p. 343. 

' A cubic iiietnj of air at the freezing-point can hold only 4*871 grammes of water- 
rai>oiir, but at 40*^ C. can take up 50*70 grammes. One cubic mile of air saturated with 
vai>our at 35'' C. will, if cooleti to 0*. deposit upwards of 140,000 tons of water as rrnin. 
Roscoe and Schorlemmer's 'Chemistr}',' i. p. 452. 



PART 1 THE OCEANS 33 



It is this vapour which chiefly absorbs radiant heat.^ It condenses into 
dew, rain, hail, and snow. In assuming a visible form, and descending 
through the atmosphere, it takes up a minute quantity of air, and of the 
different substances which the air may contain. Being caught by the 
nun, and held in solution or suspension, these substances can be best 
examined by analysing rain-water. In this way, the atmospheric gases, 
ammonia, nitric, sulphurous, and sulphuric acids, chlorides, various salts, 
solid carbon, inorganic dust, and organic matter have been detected. The 
fine microscopic dust so abundant in the air is no doubt for the most 
part due to the action of wind in lifting up the finer particles of dis- 
integrated rock on the surface of the land. Volcanic explosions sometimes 
supply prodigious quantities of fine dust There is probably also some 
addition to the solid particles in the atmosphere from the explosion and dis- 
sipation of meteorites on entering our atmosphere. To the wide diffusion 
of minute solid particles in the air great importance in the condensation 
of vapour has recently been assigned. (Book III. Part 11. Section ii.) 

The comparatively small, but by no means unimportant, proportions 
of these minor components of the atmosphere are much more liable to 
variation than those of the more essential gases. Chloride of sodium, 
for instance, is, as might be expected, particularly abundant in the air 
bordering the sea. Nitric acid, ammonia, and sulphuric acid appear most 
conspicuously in the air of towns. The organic substances present in the 
air are sometimes living germs, such as probably often lead to the pro- 
pagation of disease, and sometimes mere fine particles of dust derived 
from the bodies of living or dead organisms. ^ 

As a geological agents the atmosphere effects changes by the chemical 
reactions of its constituent gases and vapours, by its varying temperature, 
and by its motions. Its functions in these respects are described in 
Book III. Part II. Section i. 

2. The Oceans. — Rather less than three-fourths of the surface of the 
globe (or about 144,712,000 square miles) are covered by the irregular 
sheet of water known as the Sea. Within the last twenty years, much 
new light has been thrown upon the depths, temperatures, and biological 
conditions of the ocean -basins, more particularly by the Lightrdng, 
Porcupine, Challenger^ Tuscarora, Blake, Gazelle and other expeditions fitted 
out by the British, American, German, and Norwegian Governments.^ It 

^ See Tyudall's researches which established this important function of the aqueous 
mpour of the atmosphere, and their confirmation by meteorological observation. S. A. 
HilL Ptoc. Ray. Soc, xxxiii. 216, 435. 

* The air of towns is peculiarly rich in impurities, especially in manufacturing districts, 
where much coal is used. These impurities, however, though of serious consequence to 
the towns in a sanitary point of view, do not sensibly affect the general atmosphere, 
•eeing that they are probably in great measure taken out of the air by rain, even in the 
districts which produce them. They possess, nevertheless, a special geological significance, 
and in this respect, too, have important economic bearings. See on this whole subject, 
Angus Smith's * Air and Rain,' and the account of Rain in Book III. Part II. Sect. ii. 

» See Wyville Thomson, ' The Depths of the Sea, ' 1873 ; * The Atlantic, ' 1877 ; 'Report of 
Challenger Expedition,' especially the forthcoming volumes giving a summary- of results ; A. 
AgMsiz, ' Three Cruises of the Blake,' 1888 : * Den Norske Nordhavs-Expedition,' 1876-1878. 

D 



34 GEOGNOSY book ii 

has been ascertained that few parts of the Atlantic Ocean exceed 3000 
fathoms, the deepest sounding obtained there being one taken about 100 
miles north from the island of St Thomas, which gave 3875 fathoms, or 
rather less than 4| miles. The Atlantic appears to have an average 
depth in its more open parts of from 2000 to 3000 fathoms, or from 
about 2 to 3^ miles. In the Pacific Ocean H.M. Ship Challenger got 
soundings of 3950 and 4475 fathoms, or about 4| and rather more than 
5 miles. Since then the U.S. Ship Tuscarora obtained a still deeper 
sounding (4655 fathoms), to the east of the Kurile Islands. This is the 
deepest abyss yet found in any part of the ocean. But these appear to 
mark exceptionally abysmal depressions, the average depth being, as in 
the Atlantic, between 2000 and 3000 fathoms. We may therefore 
assume, as probably not far from the truth, that the average depth of the 
sea is about 2500 fathoms, or nearly 3 miles. Its total cubic contents 
will thus be about 400 millions of cubic miles. 

With regard also to the form of the bottom of the great oceans, much 
additional information has recently been obtained. Over vast areas 
in the central regions, the sea-floor appears to form great plains, with 
comparatively few inequalities, but with lines of submarine ridges, com- 
parable to chains of hills or mountains on the land. Recent soundings, 
however, taken at short distances, have revealed, in parts of the Atlantic 
that were supposed to be deep and with a tolerably uniform bottom, sub- 
marine peaks rising to within 50 fathoms from the surface.^ A vast central 
ridge has also been traced down the length of this ocean, from which a 
few lonely peaks rise above sea-level — the Azores, St. Paul, Ascension, and 
Tristan d'Acunha. In the Pacific Ocean, the lines of coral-islands appear 
to rise on submarine ridges, having a general north-westerly and south- 
easterly trend. It is significant that the islands which thus appear far 
from any large mass of land are either coral-reefs or of volcanic origin, 
and contain none of the granites, schists and other ordinary continental 
rocks. St. Helena and Ascension in the Atlantic, and the Friendly and 
Sandwich Islands in the Pacific Ocean are conspicuous examples. 

Another important result of recent deep-sea research is the determina- 
tion of the relation of mediterranean seas to the main ocean. These 
basins, such as the North, Mediterranean, and Black Seas, the Gulf of 
Mexico, Caribbean Sea, Baffin's Bay, Hudson's Bay, Sea of Okhotsk, and 
Chinese Sea, belong rather to the continental than the oceanic areas of the 
earth's surface. An elevation of a few hundred fathoms would convert 
most of them into land, with here and there deep water-filled basins. 

A question of high importance in geological enquiry is the form of the 
surface of the sea or what is usually called the sea-level. It has been 
generally assumed that this surface is stable and uniform and nearly that 
of an ellipsoid of revolution, owing its equilibrium to the force of gravity 
on the one hand and the centrifugal force of rotation on the other. But 
in recent years this conception has been called in question both by 
physicists and geologists. Observations as well as calculations have 
shown that the attraction exercised by masses of land raises the level of 

^ Times, 7th Deer. 1883. [J. Y. Buchanan.] 



PART I THE OCEANS 36 

the adjacent sea, and attempts have been made to determine how far the 
deformation thus caused departs from the mean of the theoretical ellipsoid 
of revolution. According to Bruns a continent may cause a difference of 
more than 3000 feet between the actual level of the sea and that of 
the ellipsoid. But the results of such calculations will greatly depend on 
the assmnption on which they start as to the nature of the earth's crust. 
R S. Woodward has calculated that if the continent of Europe and Asia 
be supposed to be simply a superficial aggregation of matter with a 
density as great as the parts under the sea, the elevation of sea-level at 
the centre of the continent due to attraction would amount to about 
2900 feet, but that, if the continental mass be assumed to imply a defect 
of density underneath it, the elevation of the sea at the centre of the 
continent due to attraction would be only about 10 feet.^ This subject 
is further considered in Book IIL Part I. Sect. iii. 

The water of the ocean is distinguished from ordinary terrestrial 
waters by a higher specific gravity, and the presence of so large a pro- 
portion of saline ingredients as to impart a strongly salt taste. The 
average density of sea- water is about 1*026, but it varies slightly in 
different parts even of the same ocean. According to the observations 
of J. Y. Buchanan during the Challenger expedition, some of the 
heaviest sea -water occiu^ in the pathway of the trade -winds of the 
North Atlantic, where evaporation must be comparatively rapid, a density 
of 1 '02781 being registered. Where, however, large rivers enter the sea, 
or where there is much melting ice, the density diminishes ; Buchanan 
found among the broken ice of the Antarctic Ocean that it had sunk to 
1 •02418.* A series of soundings taken during the Fega expedition in the 
Kara Sea (lat. 76° 18', long. 95" 30' E.) gave a progressive increase of 
salinity from I'l at the surface to 3*4 at 30 fathoms, the surface being 
freshened by the water poured into the sea by the Siberian rivers.^ 

The greater density of sea-water depends, of course, upon the salts 
which it contains in solution. At an early period in the earth's history, 
the water now forming the ocean, together with the rivers, lakes and 
snowfields of the land, existed as vapour, in which were mingled many 
other gases and vapours, the whole forming a vast atmosphere sur- 
rounding the still intensely hot globe. Under the enormous pressure 
of the primeval atmosphere, the first condensed water might have had a 
temperature little below the critical one.* In condensing, it would carry 
down with it many substances in solution. The salts now present in 
sea-water are to be regarded as principally derived from the primeval 
constitution of the sea, and thus we may infer that the sea has always 
been salt It is probable, however, that, as in the case of the atmosphere, 
the composition of the ocean-water has acquired its present character 

» Bruns, •Die Figur der Erde,' Berlin, 1876 ; R. S. Woodward, BuU. U, S. GeU, Surv, 
Na 48, p. 85 (1888). 

' Buchanan, Proc, Roy. Soc, (1876), voL xxiv. 

» 0. Petterason, * Vega-Expeditionens Vetenskapliga lakttagelser,* vol. ii. Stockholm, 1888. 
* Q, J. QtoL Soc xxxvi. (1880), pp. 112, 117. Fisher, • Physics of Earth's Crust,* 2nd 
«dit p. 148. 



36 GEOGNOSY book u 

only after many ages of slow change, and the abstraction of much mineral 
matter originally contained in it. There is eWdence, indeed, among 
the geological formations that large quantities of lime, silica, chlorides 
and sulphates have in the course of time been removed from the sea.^ 

But it is manifest also that, whatever may have been the original 
composition of the oceans, they have for a vast section of geological time 
been constantly receiving mineral matter in solution from the land. 
Every spring, brook, and river removes various salts from the rocks 
over which it moves, and these substances, thus dissolved, eventually 
find their way into the sea. Consequently sea -water ought to contain 
more or less traceable proportions of every substance which the terrestrial 
waters can remove from the land — in short, of probably every element pre- 
sent in the outer shell of the globe, for there seems to be no constituent of 
the earth which may not, under certain circumstances, be held in solution 
in water. Moreover, unless there be some counteracting process to remove 
these mineral ingredients, the ocean- water ought to be growing, insensibly 
perhaps, Salter, for the supply of saline matter from the land is incessant. 
It has been ascertained indeed, with some approach to certainty, that the 
salinity of the Baltic and Mediterranean is gi^adually increasing.^ 

The average proportion of saline constituents in the water of the 
great oceans far from land is about three and a half parts in every 
hundred of water.'* But in enclosed seas, receiving much fresh water, it 
is greatly reduced, while in those where evaporation predominates it is 
correspondingly augmented. Thus the Baltic water contains from one- 
seventh to nearly a half of the ordinary proportion in ocean water, while 
the Mediterranean contains sometimes one-sixth more than that propor- 
tion. Forchhammer has shown the presence of the following twenty - 
seven elements in sea-water : oxygen, hydrogen, chlorine, bromine, 
iodine, fluorine, sulphur, phosphorus, nitrogen, carbon, silicon, boron, 
silver, copper, lead, zinc, cobalt, nickel, iron, manganese, aluminium, 
magnesium, calcium, strontium, barium, sodium, and potassium.* To 

^ Sterry Hunt supix)8ed that the saline watent of North America derive their mineral 
ingredients from the sediments and precipitates of the sea in which the Paheozoic rocks 
were deposited. ' Geological and Chemical Essays,' p. 104. 

^ Paul, in Watts's * Dictionary of Chemistry',* v. p. 1020. For a detailed study of the 
Eastern Meiliterranean, see the Re]x>rts of a Commission, Denksch. Aka^. Wiss. Vienna^ 
1892, et. seq. 

' Dittmar's elaborate researches on the samples of ocean water collected by the Chal- 
lenger expedition show that the lowest percentage of salts obtained was 8*301, from the 
southern part of the Indian Ocean, south of lat. 66°, while the highest was 3*737, from the 
middle of the North Atlantic, at about lat. 23^. Some valuable results from observations 
on the waters of the North Atlantic are given by H. Tomoe and L. Schmelck in the Report 
of the Nonoegian North- Atlantic Expedition^ 1876-1878. The average proportion of salts 
was found to be from 3*47 to 3*51 per cent, the mean quantities of each constituent as 
estimated being as follow : CaCOs, 0*002 ; CaSo4, 0*1395 ; MgS04, 0*2071 ; MgClj, 0*3561 ; 
KCl, 0*0747 ; NaHCOj, 0*0166 ; NaQ, 2*682. 

* Forchhammer, PhU, Trang. civ. p. 205. According to Thorpe and Morton (Chem, Sac, 
Journ, zziv. p. 507), the water of the Irish Sea contains in summer rather more salts than in 
winter. In 1000 grammes of the summer water of the Irish Sea they found 0*04754 



PART I 



COMPOSITION OF SEA-JVATER 



37 



these may be added arsenic, lithium, caesium, rubidium, gold, and 
probably most if not all of the other elements, though in pro- 
portions too minute for detection. The chief constituents have been 
determined by Dittmar to be present in the proportions shown in the 
first column of the subjoined tables. Assuming them to occur in the 
combinations shown in the second column, they are present in the average 
ratios therein stated^ : — 



I. 



Chlorine 
Bromine 

Sulphuric acid, SO, 
Carbonic acid, CO2 
Lime, CaO . 
Magnesia, MgO . 
Potash, KO . 
Soda, Na^O . 



Snbtract Basic Oxygen equiva- ) 
lent to the Halogens J 

Totol Salts 



55-292 
0-188 
6-410 
0*152 
1-676 
6-209 
1 -332 

41-234 

12-493 
100-000 



II. 

Chloride of sodium 
Chloride of magnesium 
Sulphate of magnesia . 
Sulphate of lime * 
Sulphate of potash 
Bromide of magnesium 
Carbonate of lime 

Totol Salts 



77-758 
10-878 
4-787 
3-600 
2-465 
0-217 
0-345 

100-000 



Sea-water is appreciably alkaline, its alkalinity being due to the 
presence of carbonates, of which carbonate of lime is one/^ In addition 
to its salts it always contains dissolved atmospheric gases. From the 
researches conducted during the voyage of the BoniU in the Atlantic 
and Indian Oceans, it was estimated that the gases in 100 volumes of 
sea- water ranged from 1-85 to 3-04, or from two to three per cent. 
From observations made during the Porcupine cruise of 1868, it was 
ascertained that the proportion of oxygen was greatest in the surface 
water, and least in the bottom water. The dissolved oxygen and nitro- 
gen are doubtless absorbed from the atmosphere, the proportion so 
absorbed being mainly regulated by temperature. According to Ditt- 
mar's recent determinations, a litre of sea-water at 0° C. will take up 
15-60 cubic centimetres of nitrogen and 8 18 of oxygen, while at 30^ C. 
the proportions sink respectively to 8*36 and 4' 17. He regards the 
carbonic acid as occurring chiefly as carbonates, its presence in the free 
state being exceptional. During the voyage of the Challenger^ Buchanan 
ascertained that the proportion of carbonic acid is always nearly the 
same for similar temperatures, the amount in the Atlantic surface water, 
between 20° and 25"" C, being 00466 gramme per litre, and in the 
surface Pacific water 0*0268 ; and that sea- water contains sometimes at 
least thirty times as much carbonic acid as an equal bulk of fresh 

grammes of carbonate of lime, 0-00503 of ferrous carbonate and traces of silicic acid. For 
ezhaostive chemical investigations regarding the chemistry of ocean water con8ult Dittmar 
in vol. i. "Physics and Chemistry," Rep&rt of Voyage of the ChaUengery 1884 ; also the 
"Chemistry" part of the Iteport of the Noncegian North- Atlantic Expedition, 1876-1878. 

* Dittmar, op. cii. p. 203 et seq. For furtlier reference to the chemistry of sea-water, 
e^>ecially in connection with the action of marine organisms, see postea, p. 484. 

' Dittmar, op. cit. p. 206. 



-» i^y^jri'ti 



'ut uwsi^ae nut ^:\hws -noe^ tf ^nif 
njaui taiUK; tf -is* dasgoftsusiixisft it 'aic: 

Aji^.rtfit;? tf vuk truucnijfiija if aeh 





^'rJiutt *^ «r.ffli'jiH&ats%' uif moudc flEDttkcia Ls laie imer solid 
jj**./^. 7«: ^jfLTv yjnu'jL 'jt t: -pxi-i. SEizi£ *.":#:•«» aie saw » Tufale 
V/ u^fc; «aiC l'jrxu« 'vid^ "r* urn. Lkac. odcx^ns z^suet ukr tkia one- 



wrjT,e:wze^ V/ ^Vrjc: iiAa iLe di<:ii£fce tiervfiEz. i2ke <*z'TBicr and the 
9/r.f.e. f//>r, V^ ^^if^ra^Ji fa^iyt ion res becB ksffigrric icr tke present 
c'ijf.r.^rA^jg. *A *iji jac/i. It <:k2 be f^Kivx. bc<vieT«r. lisis partMOs of 
\i>i */ju*isj^,^A ^t 'A ^fztrecrf: g*i0ttcoai unaq^ahr. lbere> k resson to 
f^;>%^^; uAf:^^ *XaCv \tJ: ynss^ent t^rressnJ aj«fts kftTe on the whde 
*>^9«i«4 Ikiyl, fjr lAr^,, att ;ead?i. ZfeT«r beien sc L mcgyed l^neBsh deep wstcr, 
U'm* *>h^ iitt^ *A itA ^^arikct ftrmnfied formA:k<i« : A2)d tkit, on the 
'AK^r t^Mjf If *h^ '^JOkii^yAinia luiTe ^v^rs been v&s^ atcas of depreBsion. 
II;;* at^jf/j^,-! vjJJ >^ di^rUMMad in SFubsequent pases. 

ffi tl;f4; N>ir WVyrld, *b« cofitinental trend is apfjcoiinntelT north 
'4.ft/\ tt^jfiXh ; tit %hh Old World, though less distinctlr marked, it langes 
on itt^. wh^A^. tsikt TLtA wen. The intimate relation which mar be 
oS0if^irs'*^\ ^^weeri tfai^ general trend and the direction of moontain 
t:\utt0if u \As^t exhibited! by the American continent. Eoit^ and 
A1n*'/4. ut^y U; c/^rixidere^l a« forming, with Asia, the vast continental 
ut'4>M *A th<; 0\A U'orKL The existing severance of Africa and Eorope 
\h of 'y/rfjf^ratively recent rlate. On the other hand, Eorope and Asia 
w';f'; %\tA ulIwhvh v^i ^y^ntinuous as at present. Bat even where the 
t'jfiiUht^ttU 'A ih<; (M World are separated by sea, the interrening 
hollowH, ihtt%xii\i uow cj>\ht(A by ocean -water, must be regarded as 
i:wfii*uuMy fMrt of tho r^/^ntinental areas. Asia is linked with Australia 

' /'f/fT, It/fff. Mtjc. x%'\y. A*'J:ffrdiug to Mr. Tamife {Xvncegian Sorik - Atlantic 
hrjtrtl'diirn, \H7fi-7^', * * ^ ^U^ntiihiry ") mot of the carbonic acid of sea- water is in combination 
uritli n4f«\h mt Unut\MfttuUt of v^^la, Hee his memoir for an estimate of the proportion of 
fi)r lu M* wtnUrr ; hIm/i J, V. Huohanan, Xalurff zxv. p. 386. Dittmar. op. cii. p. 209. 

» O//. /;//. i*. 2ri. 

' lf\tffrftd t:nt\inuU'M have l>ecii made of the proportion of organic matter. According 
to th*- nriurfcrrli^n of L. H<:hfiielck {Xonreginn Ncrth- Atlantic Expedition^ 1876-8, Part ix. 
|f, i), iUtt pro|r//Kioii U ()'0()7Xf gramme in 100 c.c. of water. 



PART 1 TERRESTRIAL CONFIGURATION 39 

by a chain of islands. The great contrast between the Asiatic and 
Australian faunas, however, affords good grounds for the belief that, at 
least for an enormous period of time, Asia and Australia have been 
divided by an important barrier of sea. 

While any good map of the globe enables us to see at a glance 
the relative positions and areas of the continents and oceans, most 
mi^M fail to furnish any data by which the general height or volume 
of a continent may be estimated. As a rule, the mountain -chains are 
exaggerated in breadth, and incorrectly indicated, while no attempt is 
made to distinguish between high plateaux and low plains. In North 
America, for example, a continuous shaded ridge is placed down the axis 
of the continent, and marked " Rocky Mountains," while the vast level 
or gently rolling prairies are left with no mark to distinguish them from 
the maritime plains of the eastern and southern states. In reality 
there is no such continuous mountain-chain. The so-called "Rocky 
Mountains" consist of many independent and sometimes widely sepa- 
rated ridges, having a general meridional trend, and rising above 
a vast plateau, which is itself 4000 or 5000 feet in elevation. It 
is not these intermittent ridges which really form the great mass of 
the land in that region, but the widely extended lofty plateau, or 
rather succession of plateaux, which supports them. In £urope, also, 
the Alps form but a subordinate part of the total bulk of the land. 
If their materials could be spread out over the continent, it has been 
calculated that they would not increase its height more than about 
twenty-one feet.^ 

Attempts have been made to estimate the probable average height 
which would be attained if the various inequalities of the land could be 
levelled down. Humboldt estimated the mean height of £urope to be 
about 671, of Asia 1132, of North America 748, and of South America 
1151 feet^ Herschel supposed the mean height of Africa to be 1800 
feet' These figures, though based on the best data available at the time, 
are no doubt much under the truth. In particular, the average height 
assigned to North America is evidently far less than it should be ; for the 
great plains west of the Mississippi valley reach an altitude of about 5000 
feet^ and serve as the platform from which the mountain ranges rise. The 
height of Asia also is obviously much greater than this old estimate. 
G. Leipoldt has computed the mean height of Europe to be 296*838 

> M. De Lapparent (*Traite de Geologie,' 3rd edit. p. 57) gives the following estimate of 
relative heights and areas, the area below sea-level being taken as 0*6 of the whole. 
Zone I. (from sea-level to 200 metres) covers 347 per cent of the terrestrial surface. 



„ 11. 


200,, 600 




21-6 




„ in. , 


600 „ 1000 




21-4 




.. IV. , 


1000 „ 2000 




14-2 




.. V. , 


2000 „ 8000 




3-7 




,. VI. , 


3000 „ 4000 




21 




„vii. , 


, above ,, 4000 




1-7 
99-4 





» 'Aaie Centrale,' torn. i. p. 168. » * Physical Geography,' p. 119. 



40 GEOGXOSY book n 

metres (973*628 feet).^ Prof. A. De Lapparent makes the mean height 
of the land of the globe 2120 feet« and estimates the mean hei^t of 
Europe to be 958 feet, Asia 2884, Africa 1975, North America 1952, and 
South America 1762.- Dr. John Murray cmnputes these heists as 
follows: Europe 939, Asia 3189, Africa 2021, North America 1888, 
South America 2078, Australia 805 feet, general mean hei^t of land 
2252 feet.^ It is of some consequence to obtain as near an approxi- 
mation to the truth in this matter as maj be possible, in order to 
furnish a means of comparison between the relatiTe bulk of different 
continents, and the amount of material on which geological changes can 
be effected. 

The highest elevation of the surface of the land is the summit of 
Mount Everest, in the Himalaya range (29,000 feet); the deepest 
depression not covered by water is that of the shores of the Dead Sea 
(1300 feet below sea- level). There are, however, many subaqueous 
portions of the land which sink to greater depths. The bottom of the 
Caspian Sea, for instance, lies about 3000 feet below the general sea-leveL 
The vertical difference between the highest point of the land and the 
maximum known depth of the sea is 56,932 feet or nearly 1 1 miles. 

There are two conspicuous junction-lines of the land with its over- 
lying and surrounding envelopes. First, with the Air, expressed by 
the contours or relief of the land. Second, with the Sea, expressed by 
coastrlines. 

(1.) Contours or Relief of the Land. — While the surface 
of the land presents endless diversities of detail, its leading features 
may be generalised as mountains, table-lands, and plains. 

Mountains. — The word "mountain" is, properly speaking, not a 
scientific term. It includes many forms of ground utterly different from 
each other in size, shape, structure, and origin. It is popularly applied 
to any considerable eminence or range of heights, but the height and size 
of the elevated ground so designated vary indefinitely. In a really 
mountainous country the word would be restricted to the loftier masses 
of ground, while such a word as hill would be given to the lesser heights. 
But in a region of low or gently undulating land, where any conspicuous 
eminence becomes important, the term mountain is lavishly used. In 
Flastern America this habit has been indulged in to such an extent, that 
what are, so to speak, mere hummocks in the general landscape, are 
dignified by the name of mountains. 

It is hanlly ix)ssible to give a precise scientific definition to a term so 
vaguely employed in ordinary language. When a geologist uses the word, 
he must either be content to take it in its familiar vague sense, or must 
add some ])hrase defining the meaning which he attaches to it. He finds 

' ' Die Mittlere Uiihe Europas,' Leipzig, 1874. lu tLis work the mean height of 
Hwitzerland is put down as 1299*91 metres ; Spanish peninsula, 700*60 ; Austria, 517*87 ; 
Italy, 517*17: Scandinavia, 428 10; France, 393*84: Great BriUiu, 217*70; German 
Kmpire, 213*66 ; Russia, 167*09 ; Belgium, 163*36 ; Denmark (exclusive of Iceland), 35-20 ; 
the Netherlands (exclusive of Luxenil>ourg and the tracts below sea-level), 9*61. 

2 'Traits,' p. 56. » .Scottish Gfog. Mag, iv. (1888), 23. 



PART I TYPES OF MOUNTAINS 41 

that there are three leading and totally distinct types of elevation which 
are all popularly termed mountains. 1. Single eminences, standing alone 
upon a plain or table-land. This is essentially the volcanic type. The 
huge cones of Vesuvius, £tna, and Teneriffe, as well as the smaller ones 
so abundant in volcanic districts, are examples of it There occiu-, 
however, occasional isolated eminences that stand up as remnants of once 
extensive rock -formations. These have no real analogy with volcanic 
elevations, but should be classed luider the next type. The remarkable 
huUes of Western America are good illustrations of them. 2. Groups of 
eminences connected at the sides or base, often forming lines of ridge 
between divergent valleys, and owing their essential forms not to under- 
ground structure so much as to superficial erosion. Many of the more 
ancient uplands, both in the Old World and the New, furnish examples 
of this type, such as the Highlands of Scotland, the hills of Cumberland 
and Wales, the high grounds between Bohemia and Bavaria, the Lauren- 
tide Mountains of Canada, and the Green and White Mountains of New 
England. 3. Lines of lofty ridge rising into a succession of more or less 
distinct summits, their general external form having relation to an internal 
plication of their component rocks. These linear elevations, whose 
existence and trend have been determined immediately by subterranean 
movement, are the true mountain-ranges of the globe. They may be 
looked upon as the crests of the great waves into which the crust of the 
earth has been thrown. All the great mountain-lines of the world belong 
to this type. 

Leaving the details of mountain -form to be described in Book VIL, 
we may confine our attention here to a few of the more important 
general features. In elevations of the third or true mountain type, 
there may be either one line or range of heights, or a series of parallel 
and often coalescent ranges. In the Western Territories of the United 
States, the vast plateau has been, as it were, wrinkled by the uprise of 
long intermittent ridges, with broad plains and basins between them. 
Each of these forms an independent mountain-range. In the heart of 
Europe, the Bernese Oberland, the Pennine, Lepontine, Rhaetic, and 
other ranges form one great Alpine chain or system. 

In a great mountain-chain, such as the Alps, Himalayas, or Andes, 
there is one general i>ersistent trend for the successive ridges. Here 
and there, lateral offshoots may diverge, but the dominant direction of 
the axis of the main chain is generally observed by its component ridges 
until they disappear. Yet while the general parallelism is preserved, no 
single range may be traceable for more than a comparatively short dis- 
tance ; it may be found to pass insensibly into another, while a third may 
be seen to begin on a slightly different line, and to continue with the 
same dominant trend until it in turn becomes confluent. The various 
ranges are thus apt to assume an arrangement en Echelon. 

The ranges are separated by lon^tudinal valleys, that is, depressions 
coincident with the general direction of the chain. These, though 
sometimes of great length, are relatively of narrow width. The valley 
of the Rh6ne, from the source of the river down to Martigny, offers an 



42 GEOGNOSY book ii 

excellent example. By a second series of valleys the ranges are trenched, 
often to a great depth, and in a direction transverse to the general trend. 
The Rhdne famishes also an example of one of these transverse valleys, 
in its course from Martigny to the Lake of Greneva. In most mountain 
regions, the heads of two adjacent transverse valleys are often connected 
by a depression or pass (co/, joch), 

A large block of mountain ground, rising into one or more domi- 
nant summits, and more or less distinctly defined by longitudinal 
and traverse valleys, is termed in French a massif — a word for which 
there is no good £nglish equivalent Thus in the Swiss Alps we 
have the massifs of the Glamisch, the Todi, the Matterhom, the Jung- 
frau, &c 

Very exaggerated notions are common regarding the angle of 
declivity in mountains. Sections drawn across any mountain or 
mountain-chain on a true scale, that is, with the length and height on 
the same scale, bring out the fact that^ even in the loftiest mountains, 
the breadth of base is always very much greater than the height 
Actual vertical precipices are less frequent than is usually supposed, 
and even when they do occur, generally form minor incidents in the 
declivities of mountains. Slopes of more than 30° in angle are likewise 
far less abundant than casual tourists believe. £ven such steep 
declivities as those of 38^ or 40° are most frequently found as 
talus-slopes at the foot of crumbling cliffs, and represent the angle 
of repose of the disintegrated debris. Here and there, where the 
blocks loosened by weathering are of large size, they may accumulate 
upon each other in such a manner that for short distances the average 
angle of declivity may mount as high as 65°. But such steep slopes 
are of limited extent. Declivities exceeding 40 , and bearing a large 
proportion to the total dimensions of hill or mountain, are always found 
to consist of naked solid rock. In estimating angles of inclination from 
a distance, the student will learn by practice how apt is the eye to be 
deceived by perspective and to exaggerate the true declivity, sometimes 
to ^stake a horizontal for a highly inclined or vertical line. The 
mountain outline shown in Fig. 2 presents a slope of 25° between a and 
b, of 45° between b and c, of 17° between c and ^, of 40° between d and e, 
and of 70° between e and/. At a great distance, or with bad conditions 
of atmosphere, these might be believed to be the real declivities. Yet if 
the same angles be observed in another way (as on a cottage roof at B), 
we may leani that an apparently inclined surface may really be 
horizontal (as from a to b and from c to d), and that by the effect 
of perspective, slopes may be made to appear much steeper than they 
really are.^ 

Much evil has resulted in geological research from the use of 
exaggerated angles of slope in sections and diagrams. It is therefore 
desirable that the student should, from the beginning, 'accustom himself 

' Mr. Ruskin has well illustrated this point. See ' Modem Painters,' vol. iv. p. 183, 
whence the illustrations in the text are taken. 



PA«T I TABLE-LANDS 

to the drawing of outlines as nearly as possible on a true scale, 
accompanying section of the Alps by De la Beche (Fig. 3) 
is of interest in this respect, as one of the earliest iUus- ^ 
tntions of the advantage of constructing geological sections 




OD a true scale as to the relative proportions of height and 
length.^ 

Xaite-laiub or Plateaux are elevated regions of flat or 
undulating country, rising to heights of 1000 feet and up- 
wards above the level of the sea. They are sometimes 
bordered with steep elopes, which descend from their edges, 
as the table-land of the Spanish peninsula does into the sea. 
In other cases, they gradually sink into the plains and have 
no definite boundaries; thus the prairie- land west of the 
Missouri slowly and imperceptibly ascends until it becomes 
a vast plateau from 4000 to 5000 feet above the sea. Occa- 
sionally a high table-land is encircled with lofty mountains, as 
in those of Quito and Titicaca among the Andes, and that of 
the heart of Asia ; or it forms in itself the platform on 
which lines of mountains stand, as in North America, where 
the ranges included within the Bocky Mountains reach 
elevations of from 10,000 to 14,000 feet above the sea, but 
not more than from 5000 to 10,000 feet above the table-land. 
Two types of table-land structure may be observed. I. 
Table-lands consisting of level or gently undulated sheets of 
rock, the general surface of the country corresponding with 
that of the stratification. The Rocky Mountain plateau 
is an example of this type, which may be called that 

> 'Sections and Views, HlostratjTe of Geological Phenoniean.' 1830. Oeo 
V.U6. 



p 



44 GEOGNOSY book ii 



of Deposit, for the flat strata have been equably upraised nearly in 
the position in which they were deposited. 2. Table-lands formed out 
of contorted, crystalline, or other rocks, which have been planed down 
by superficial agents. This type, where the external form is independent 
of geological structure, may be termed that of Erosion. The fjdds of 
Norway are portions of such a table-land. In proportion to its antiquity, 
a plateau is trenched by ninning water into systems of valleys, until in 
the end it may lose its plateau character and pass into the second t3rpe 
of mountain-ground above described. This change has largely altered 
the ancient table-land of Scandinavia, as will be illustrated in Book VII. 

Plains are tracts of lowland (under 1000 feet in height) which 
skirt the sea-board of the continents and stretch inland uj) the river 
valleys. The largest plain in the world is that which, beginning in the 
centre of the British Islands, stretches across Europe and Asia. On the 
west, it is bounded by the ancient table-lands of Scandinavia, Scotland 
and Wales on the one hand, and those of Spain, France and Germany 
on the other. Most of its southern boundary is formed by the vast belt 
of high ground which spreads from Asia Minor to the east of Siberia. 
Itfi northern margin sinks beneath the waters of the Arctic Ocean. This 
vast region is divided into an eastern and western tract by the low chain 
of the Ural Mountains, south of which its general level sinks, until 
underneath the Caspian Sea it reaches a depression of about 3000 feet 
below searlevel. Along the eastern sea-board of America lies a broad 
belt of low plains, which attain their gi-eatest dimensions in the regions 
watered by the larger rivers. Thus they cover thousands of square 
miles on the north side of the Gulf of Mexico, and extend for hundreds 
of miles up the valley of the Mississippi. Almost the whole of 
the valleys of the Orinoco, Amazon and I^a Plata is occupied with vast 
plains. 

From the evidence of upraised manne shells, it is certain that large 
portions of the great i)lain of the Old World comparatively recently 
formed part of the sea-floor. It is likewise })robable that the beds of 
some enclosed sea-basins, such as that of the North Sea, have formerly 
been plains of the dry land. ^ 

It is obvious, from their distribution along river-valleys, and on the 
areas between the base of high gi-ounds and the sea, that plains are 
essentially areas of deposit. They are the tracts that have received the 
detritus washed down from the slopes above them, whether that detritus 
has originally accumulated on the land or below the sea. Their surface 
presents everywhere loose sandy, gravelly, or clayey formations, indica- 
tive of its comjyaratively recent subjection to the operation of running 
water. y 

(2.) Coast-lines. — A mere inspection of a map of the globe brings 
before the mind the striking differences which the masses of land present 
in their line of junction with the sea. As a rule, the southern con- 
tinents possess a more uniform unindented coast-line than the northern. 
It has been estimated that the ratios between area and coast-line among 
the different continents, stand approximately as in the following table : — 



PART 1 COAST-LINES OF CONTINENTS 45 

r Europe has 1 geographical mile of coast-line to 143 st^uare miles of surface. 
Korthem. -I North America ,, ,, 265 

I Asia, including the islands ,, 469 



(Africa „ „ 895 



Southern. -< South America ,, ,, 434 ,, 

V Australia ,, t, 332 ,, 

In estimating the relative potency of the sea and of the atmospheric 
agents of disintegration, in the task of wearing down the land, it is 
eyidently of great importance to take into account the amount of surface 
respectively exposed to their operations. Other things being equal, 
there is relatively more marine erosion in Europe than in North America. 
But we require also to consider the nature of the coastnline, whether flat 
and alluvial, or steep and rocky, or with some intermediate blending of 
these two characters. By attending to this point, we are soon led to 
observe such great differences in the character of coast-lines, and such 
an obvious relation to differences of geological structure, on the one 
hand, and to diversities in the removal or deposit of material, on the 
other, as to suggest that the present coast-lines of the globe cannot be 
aboriginal, but must be referred to the operation of geological agents still 
at work. This inference is amply sustained by more detailed investi- 
gation. While the general distribution of land and water must un- 
doubtedly be assigned to terrestrial movements affecting the solid globe, 
the present actual coasts of the land have chiefly been produced by 
local causes. Headlands project from the land because, for the most 
part, they consist of rock which has been better able to withstand the 
shock of the breakera. Bays and creeks, on the other hand, have been cut 
by the waves out of less durable materials. Again, by the sinking of 
land, ranges of hills have become capes and headlands, while the valleys 
have passed into the condition of bays, inlets, or fjords. By the uprise of 
the sea-bottom, tracts of low alluvial ground have been added to the land. 
Hence, speculations as to the history of the elevation of the land, 
based merely upon inferences from the form of coast-lines as expressed 
upon ordinary maps, to be of real service, demand a careful scrutiny 
of the actual coast-lines, and an ^amount of geological investigation 
which would require long and patient toil for its accomplishment. 

Passing from the mere external form of the land to the composition 
and structure of its materials, we may begin by considering the general 
density of the entire globe, computed from observations and compared 
with that of the outer and accessible portion of the planet. Reference 
has already been made to the comparative- density of the earth among 
the other members of the solar system. In inquiries regarding the 
history of our globe, the density of the whole mass of the planet, as 
compared with water — the standard to which the specific gravities of 
terrestrial bodies are referred — is a question of prime importance. 
Various methods have been employed for determining the earth's 
density. The deflection of the plumb-line on either side of a mountain 
of known structure and density, the time of oscillation of the pendulum 



46 



GEOGNOSY 



BOOK U 



at great heights, at the sea-level, and in deep mines, and the comparative 
force of gravitation as measured by the torsion balance, have each been 
tried with the following various results : — 

Plumb-line ex])erimeiits on Schichallien (Maskelyne and Playfair) 

gave as the mean density of the earth 4-713 

Do. on Arthur's Scat, Edinburgh (James) 5-316 

Pendulum experiments on Mont Cenis (Carlini and Giulio) . . 4-960 

Do. in Hartou coal-pit, Newcastle (Airy) 6-565 

Torsion lialance experiments (Cavendish, 1798) 5-480 

Do. do. (Reich, 1838) 5-49 

Do. do. (Baily, 1843) 5-660 

Do. do. (Comu and Bailie, 1872-3) . . . 5-50-5-56 

Though these observations are somewhat discrepant, we may feel 
satisfied that the globe has a mean density neither much more nor much 
less than 5*5 ; that is to say, it is five and a half times heavier than one 
of the same dimensions formed of pure water. Now the average 
density of the materials which compose the accessible portions of the 
earth is between 2*5 and 3 ; so that the mean density of the whole globe 
is about twice as much as that of its outer part. We might, therefore, 
infer that the inside consists of much heavier materials than the outside, 
and consequently that the mass of the planet must contain at least two 
dissimilar portions — an exterior lighter crust or rind, and an interior 
heavier nucleus. But the effect of pressure must necessarily increase 
the specific gravity of the interior, as will be alluded to further on. 

§ 2. The Crust. — It was formerly a prevalent belief that the exterior 
and interior of the globe differed from each other to such an extent that, 

while the outer parts were 
cool and solid, the vastly more 
enormous inner intensely hot 
part was more or less completely 
liquid. Hence the term " crust " 
was applied to the external rind 
in the usual sense of that word. 
This crust was variously com- 
puted to be ten, fifteen, twenty, 
or more miles in thickness. In 
the accompanying diagram (Fig. 
4), for example, the thick line 
forming the circle represents a 
relative thickness of 100 miles. 
There are so many proofs of 
enormous and "wide-spread cor- 
r.. . o . r^ * ***. 1^ -*u ,^«:i *v. 1 rugation of the materials of the 

Fig. 4.— Supposed Cruat of the Earth, 100 Miles thick. ^ , , , _ . 

earth s outer layers, and such 
abundant traces of former volcanic action, that geologists have naturally 
regarded the doctrine of a thin crust over a liquid interior as necessary 
for the explanation of a large class of terrestrial phenomena. For 
reasons which will be afterwards given, however, this doctrine has been 




PART I DISTRIBUTION OF TERRESTRIAL DENSITY 47 



opposed by eminent physicists, and is now abandoned by most geologists. 
Nevertheless the term " crust " continues to be used, apart from all theory 
regarding the nucleus, as a convenient word to denote those cool, upper 
or outer layers of the earth's mass in the structure and history of which, 
as the only portions of the planet accessible to human observation, lie 
the chief materials of geologic^d investigation. The chemical and mineral 
constitution of the crust is fully discussed in later pages (p. 60 et seq,) 

§ 3. The Interiop op Nucleus. — Though the mere outside skin of 
our planet is all with which direct acquaintance can be expected, the 
irregular distribution of materials beneath the crust may be inferred 
from the present distribution of land and water, and the observed 
differences in the amount of deflection of the plumb-line near the sea and 
near mountain chains. The fact that the southern hemisphere is almost 
wholly covered with water, appears only explicable, as already remarked, 
on the assumption of an excess of density in the mass of that half of the 
planet. The existence of such a vast sheet of water as that of the 
Pacific Ocean is to be accounted for, says Archdeacon Pratt, by the 
presence of " some excess of matter in the solid parts of the earth between 
the Pacific Ocean and the earth's centre, which retains the water in its 
place, otherwise the ocean would flow away to the other parts of the 
earth." ^ The same writer points out that a deflection of the plumb-line 
towards the sea, which has in a number of cases been observed, indicates 
that " the density of the crust beneath the mountains must be less than 
that below the plains, and still less than that below the ocean-bed."^ 
Apart, therefore, from the depressions of the earth's surface, in which the 
oceans lie, we must regard the internal density, whether of crust or 
nucleus, to be somewhat irregularly arranged, — there being an excess of 
heavy materials in the water-hemisphere, and beneath the ocean-beds as 
compared with the continental masses. 

It has been argued from the difference between the specific gravity 
of the whole globe and that of the crust, that the interior must consist 
of heavier material, and may be metallic. But the effect of the enormous 
internal presisure, it might be supposed, should make the density of the 
nucleus much higher, even if the interior consisted of matter which, on 
the surface, would be no heavier than that of the cnist. In fact, we 
mighty on the contrary, argue for the probable comparative lightness 
of the substance composing the nucleus. That the total density of the 
planet does not greatly exceed its observed amount, may indicate that 
some antagonistic force counteracts the effect of pressure. The only force 
we can suppose capable of so acting is heat, though to what extent this 
counterbalancing takes place is still unknown. It must be admitted 
that we are still in ignorance of the law that regulates the compression 
of solids under such vast pressure as must exist within the earth's 
interior. We know that gases and vapours may be compressed into 

1 'Fignra of the Earth/ 4th edit. p. 236. 

» Op. «<. p. 200. See also Herschel, 'Phys. Geog/ § 13 ; 0. Fisher, Cambridge PhU. 
Trans, xii part ii. ; 'Physics of the Earth's Crust,' p. 75. PhU. Mag. July, 1886. Faye, 
CoK^fUi rtndus, cii (1886), p. 651. 



48 GEOGNOSY book ii 

liquids, sometimes even into solids, and that in the liquid condition 
another law of compressibility begins. We know also from experiment 
that some substances have their melting-point raised by pressure.^ It 
may be that the same effect takes place within the earth ; that pressure 
increasing inward to the centre of the globe, while augmenting the 
density of each successive shell, may retain the whole in a solid condition, 
yet at temperatures far above the normal melting-points at the surface. 
Hence, on this view of the matter, it is conceivable that the difference 
between the density of the whole globe and that of the crust may be due 
to pressure, rather than to any essential difference of composition. Laplace 
proposed the hypothesis that the increase of the. square of the density is 
proportional to the increase of the pressure, which gives a density of 8-23 
at half the terrestrial radius and of 10-74 at the centre. From another 
law proposed by Prof. Darwin, the density at half the radius is only 7-4 
but thence towards the centre increases rapidly up to infinity.^ Dr. Pfaff 
believes that the mean terrestrial density of 5*5 is not incompatible with 
the notion that the whole globe consists of materials of the same density 
as the rocks of the crust.^ It is possible that the gases dissolved in the 
hot magma of the nucleus, with their very high tension, may counteract 
the effects of compression and thus reduce density. 

Analogies in the solar system, however, as well as the actual struc- 
ture of the rocky cnist of the globe, suggest that heavier metallic 
ingredients possibly predominate in the nucleus. If the materials of the 
globe were once, as they are believed to have been, in a liquid condition, 
they would then doubtless be subject to internal arrangement, in accord- 
ance with their relative specific gravities. We may conceive that, as in 
the case of the sun, as well as of the solar system generally (ante, p. 9), 
there would be, so long as internal mobility lasted, a tendency in the 
denser elements of oiu* planet to gravitate towards the centre, in tfie 
lighter to accumidate outside. That a distribution of this nature has 
certainly taken place to some extent, is evident from the structure of the 
envelopes and crust. It is what might be expected, if the constitution of 
the globe resembles, on a small scale, the larger planetary system of 
which it forms a part. The existence even of a metallic interior has 
been inferred from the metalliferous veins which traverse the crust, and 
which are commonly supposed to have been filled from below. 

Evidence of Internal Heat. — In the evidence obtainable as to 
the former history of the eiirth, no fact is of more importance than the 
existence of a high temperature beneath the crust, which has now been 
placed beyond all doubt. This feature of the planet's organisation is 
made clear by the follo\ving proofs : — 

(1.) Volcarwes. — In many regions of the earth's surface, openings exist 

^ Under a pressure of 792 atmospheres, spermaceti has its melting-point raised from 51*^ 
to 80*2^, and wax from 64 S* to 80-2". 

- See Fisher * Physics of Earth's Crust/ 2nd edit. chap. ii. Legendre supposed that the 
density being 2'5 at the surface, it is 8*5 at half the length of the radius and 11*3 at the 
centre. More recently K Roche calculated these densities to be 2 '1, 8 '5, and 10 '6 respectively. 

^ ' Allgemeine Geologie als ezacte Wissenschaft,' p. 42. 



PAKT I THE EARTH'S INTERNAL HEAT 49 

from which steam and hot vapours, ashes and streams of molten rock, 
are from time to time emitted. The abundance and wide diffusion of 
these openings, inexplicable by any mere local causes, must be regarded 
as indicative of a very high internal temperature. If to the still active 
vents of eruption, we add those which have formerly been the channels 
of communication between the interior and the surface, there are 
perhaps few large regions of the globe where proofs of volcanic 
action cannot be found. Everywhere we meet with masses of molten 
rock which have risen from below, as if from some general reservoir. 
The phenomena of active volcanoes are fidly discussed in Book III. Part I. 

(2.) Hot Springs, — Where volcanic eruptions have ceased, evidence of a 
high internal temperature is still often to be found in springs of hot water 
which continue for centuries to maintain their heat. Thermal springs, 
however, are not confined to volcanic districts. They sometimes rise 
even in regions many hundreds of miles distant from any active volcanic 
vent The hot springs of Bath (temp. 120° Fahr.) and Buxton (temp. 
82° Fahr.) in England are fully 900 miles from the Icelandic volcanoes on 
the one side, and 1100 miles from those of Italy and Sicily on the other. 

(3.) Borings, IVells and Mines. — The influence of the seasonal changes 
of temperature extends downward from the surface to a depth which 
\'aries with latitude, with the thermal conductivity of soils and rocks, 
and perhaps with other causes. The cold of winter and the heat of 
summer may be regarded as following each other in successive waves 
downward, until they disappear along a limit at which the temperature 
remains constant This zone of invariable temperature is commonly 
believed to lie at a depth of somewhere between 60 and 80 feet in tem- 
perate regions. At Yakutsk in Eastern Siberia (lat. 62"* N.), however, 
as shown in a well-sinking, the soil is permanently frozen to a depth of 
about 700 feet^ In Java, on the other hand, a constant temperature is 
said to be met with at a depth of only 2 or 3 feet^ 

It is a remarkable fact, now verified by observation all over the 
world, that below the limit of the influence of ordinary seasonal changes 
the temperature, so far as we yet know, is nowhere found to diminish 
downwards. It always rises ; and its rate of increment never falls much 
below the average. The only exceptional cases occur under circum- 
stances not difficult of explanation. On the one hand, the neighbourhood 
of hot-springs, of large masses of lava, or of other manifestations of 
volcanic activity, may raise the subterranean temperatiu*e much above 
its normal condition ; and this augmentation may not disappear for many 
thousand yeara after the volcanic activity has wholly ceased, since the 
cooling down of a subterranean mass of lava must necessarily be a very 
slow process. Lord Kelvin has even proposed to estimate the age of sub- 
terranean masses of intrusive lava from their excess of temperature above 
the normal amount for their isogeotherms (lines of equal earth -tem- 
perature), some probable initial temperature and rate of cooling being 
assumed. On the other hand, the spread of a thick mass of snow and ice 



1 



Helmcrsen, Brit, Assoc. Rep. 1871, p. 22. See vol. for 1886, p. 271. 

2 Junghuhn's * Java,' ii. p. 771. 
E 



50 GEOGNOSY book ii 

over any considerable area of the earth's surface, and its continuance 
there for several thousand years, would so depress the isogeotherms that, 
for many centuries afterwards, there would be a fall of temperature for a 
certain distance downwards. At the present day, in at least the more 
northerly parts of the northern hemisphere, there are such evidences of a 
former more rigorous climate, as in the well-sinking at Yakutsk just 
referred to.^ Lord Kelvin (Sir W. Thomson) ^ has calcidated that any 
considerable area of the earth's surface covered for several thousand years 
by snow or ice, and retaining, after the disappearance of that frozen 
covering, an average surface temperature of IS"" C, "would during 900 
years show a decreasing temperature for some depth down from the surface, 
and 3600 years after the clearing away of the ice would still show residual 
effect of the ancient cold, in a half rate of augmentation of temperature 
downwards in the upper strata, gradually increasing to the whole normal 
rate, which would be sensibly reached at a depth of 600 metres." 

Beneath the limit to which the influence of the changes of the seasons 
extends, observations all over the globe, and at many diflferent elevations, 
give a rate of increase of temperature downwards, or " temperature gra- 
dient," which has been usually taken to be 1" Fahr. for every 50 or 60 
feet of descent, this computation being based especially on observations 
in deep mines and borings. Professor Prestwich concluded from a large 
series of observations collated by him, that the average increment might 
be taken at 1" Fahr. for every 45 feet.^ Observations taken in the 
extraordinarily deep boring at Schladebach, near Diirrenberg, showed 
that in a depth of 5736 feet the average rise of temperature was 1° 
Fahr. for every 65 feet.* According to data collected by a Committee 
of the British Association, the average gradient appears to be 1^ Fahr. 
for every 64 feet, or ^^^ of a degree per foot. 

Isogeotherms near the surface follow approximately the contours of 
the surface, but are flatter than these, and '* their flattening increases as 
we pass to lower ones, until at a considerable depth they become sensibly 
horizontal planes. The temperature gradient is consequently steepest 
beneath gorges and least steep beneath ridges.' 



»5 



^ Professor Prestwich {Inaugural Ledurtt 1875, p. 45) has suggested that to the more 
rapid refrigeration of the earth's surface during this cold period, and to the consequent 
depression of the subterranean isothermal lines, the alleged present comparative quietude 
of the volcanic forces is to be attiibuted, the internal heat not having yet recovered its 
dominion in the outer crust. 

"^ Brit, Assoc, Repcn'ts^ 1876, Sections, p. 3. ^ Proc, Roy, Soc. xli. (1885), p. 55. 

* Brit. Assoc. 1889. Report of Underground Temperature Committee. 

* J. D. Everett, Brit. Assoc. 1879, Sections, p. 345. Compare also the elaborate 
observations made in the St Gothard Tunnel, F. StapfT, * Rapports, Conseil Fed. 
St. Gothard,' vol. viii., and 'Geologische Durchschnitte des Gothard Tunnels;* 'J^tude de 
rintiuence de la Clialeur de I'lnt^rieur de la Terre,' &c., Becue Univ. Mines, 1879-80. 
Min. Proc* X. EngUind Inst. Mining -Mechan. Engin. xxxii. (1883), p. 19. 'Reports 
of Committee on Underground Temperature,' Brit. Assoc. Hep. from 1868 onwards, with 
summary of results in tlie volume for 1882. A voluminous and valuable collection of data 
bearing on this subject was compiled by Professor Prestwich and is published in Proc. Boy. 
Soc. xli. (1885), p. 1. 



PART I 



THE EARTH'S INTERNAL HEAT 



51 



Irregularities in the Downward Increment of Heat. 
— While there is everywhere a progressive increase of temperature 
downwards, its rate is by no means uniform. The more detailed 
obeervations which have been made in recent years have brought to 
light the important fact that considerable variations in the rate of 
increase take place, even in the same bore. The temperatures obtained 
at different depths in the Rose Bridge colliery shaft, Wigan, for instance, 
read as in the following columns : — 



Depth ill 
Yardtt. 

558 

605 

630 

663 

671 

679 

734 



TempeTBture 

(Fahr.) 

78 


Depth in 
Yards. 
745 


80 


761 


83 


775 


85 


783 


86 


800 


87 


806 


88i 


815 



Temperature 
(Fahr.) 

89 

90i 

914 

92 

93 

93J 

94 



At La Chapelle, in an important well made for the water-supply of 
Paris, observations have been taken of the temperature at different 
depths, as shown in the subjoined table : ^ — 



Depth ill 


Temperature 


Depth in 


Temiwraturt 


Metres. 


(Fahr.) 


Metres. 


(Fahr.) 


100 


59-5 


500 


72-6 


200 


61-8 


600 


75-0 


300 


65-5 


660 


76 


400 


69-0 







In drawing attention to the foregoing temperature-observations at the 
Ro6e Bridge colliery — the deepest mine in Great Britain — Professor Everett 
points out that, assuming the surface temperature to be 49° Fahr., in the 
first 558 yards, the rate of rise of temperature is 1^ for 57*7 feet; in the 
next 257 yards it is 1° in 48*2 feet; in the portion between 605 and 671 
yards — a distance of only 198 feet — it is l"" in 33 feet; in the lowest 
portion of 432 feet it is 1** in 54 feet.- When such irregularities occur 
in the same vertical shaft, it is not surprising that the average should 
vary so much in different places. 

There can be little doubt that one main cause of these variations 
is to be sought in the different thermal conductivities of the rocks of 
the earth's crust. The first accurate measurements of the conductini^ 
powers of rocks were made by the late J. D. Forbes at Edinburgh 
(1837-1845). He selected three sites for his thermometers, one in 
"trap-rock" (a porphyrite of Lower Carboniferous age), one in loose 
sand, and one in sandstone, each set of instruments being sunk to depths 
of 3, 6, 12 and 24 French feet from the surface. He found that the 
wave of summer heat reached the bulb of the deepest instrument (24 
feet) on 4th January in the trap-rock, on 25th December in the sand 

» BrU. Assoc. Rep. 1873, Sectiou8, p. 254. ^ Brit, Assoc. Hep. 1870, Sections, p. 31. 



62 GEOGNOSY book ii 

<ind on 3r(l November in the sandstone, the trap-rock l)eing the worst 
conductor and the solid sandstone by far the best* 

As a rule, the lighter and more porous rocks offer the greatest 
resistance to the passage of heat, while the more dense and crystalline 
offer the least resistance. The resistance of opaque white quartz is 
expressed by the number 114, that of Itasalt stands at 273, while that <A 
cannel coal stands very much higher at 1538, or more than thirteen 
times that of quartz.^ 

It is c^'ident also, from the texture and structure of most rocks, that 
the conductivity must vary in different directions through the same 
mass, heat being more easily conducted along than across the "grain," 
the bedding, and the other numerous divisional surfaces. Experiments 
have been made to determine these variations in a number of rocks. 
Thus, the conductivity in a direction transverse to the divisional planes 
being taken as unity, the conductivity parallel with these planes was 
found in a variety of magnesian schist to lie 4028. In certain slates 
and schistose rocks from central France, the ratio varied from 1 : 256 to 
1 : 3'9d2. Hence in such fissile rocks as slate and mica-schist, heat may 
travel four times more easily along the planes of cleavage or foliation 
than across them.^ 

In reasoning upon the discrepancies in the rate of increase of sub- 
terranean temperatures, we must also bear in mind that convection by 
])ercolating streams of water must materially affect the transference <^ 
heat from below.^ Certain kinds of rock are more liable than others to 
1)0 charged with water, and, in almost every boring or shaft, one or mrav 
horizons of such water-bearing i-ocks are met with. The effect of 
interstitial water ta to diminish thermal resistance. Dry red brick has 
its resistance lowered from 680 to 405 by being thoroughly soaked in 
water, its conductivity being thus increased 68 per cent, A piece of 
sandstone has i\& conductivity heightened to the extent of S per cent by 
being wetted." 

Mallet contended that the variations in the amount of increase 
in subterranean temperature arc t^X) great to j^ermit us to believe tham 
to be due merely to differences in the transmission of the general 
internal heat, and that they point to local accessions of heat arising from 
transformation of the mechanical work of compression, which i> due to 
the constant cooling and contraction of the globe." But it nuj bs 

' Tram. Rvy. Soc. Edia, nvi. p. 211. 

' UeTBchel «inl Lsbour (Britisb Ansociation Cominittei: on Tlieriijnl Conilin 
Rocks), Bril. At»"c Hep. 1875, p. 59. The fiual Report U in Ihe toI. for 1881. 

^ " Report of Committee on Thermal Cooductivities of Biek, " Brit. Auoe. ftrji. 1876, 
!>. ttl. JBDnetttti, BuU. Soc. QM. Frana (April-June, 1874), li. p. 284. Thb obstrrer 
haa carried out a aeries of detailed reBearclies on tbe projiaiiistiun of lieat Uiroiigli rocVi 
which will be foaiid in BuU. Soc GM. Franar, tomn i.-ix. (3r<1 &er'm}. 

* In the great bore of Sperenberg (4172 feet, entinlf iu rock-utlt, eic*|it tbe Snl SU 
feet] there ia evidence thut the water near the top ii mrnied 4}° ~ ' ' 
Bril. A'sv. 1882, p, 78. 

' Hentchel and Leboiir, Brit, Aaoe. Sep. IS75, p. 68. 

' "Voleanic Energy," Phil. Tram. 1876. 



^ 



PART I THE EARTH'S INTERIOR 53 

replied that these variations are not greater than, from the known diver- 
gences in the conductivities of rocks, they might fairly be expected to be. 
Probable Condition of the Earth's Interior. — Various theo- 
ries have been propounded on this subject. There are only three which 
merit serious consideration. (1.) One of these supposes the planet to 
consist of a solid cnist and a molten interior. (2.) The second holds 
thsLty with the exception of local vesicular spaces, the globe is solid and 
rigid to the centre. (3.) The third contends that while the mass of the 
globe is solid, there lies a liquid substratum beneath the crust 

1. The arguments in favour of intermtl liquidity may be summed up 
as follows, (a.) The ascertained rise of temperature inwards from the 
surface is such that, at a very moderate depth, the ordinary melting- 
point of even the most refractory substances would be reached. At 20 
miles the temperature, if it increases progressively, as it does in the 
depths accessible to observation, must be about 1760° Fahr. ; at 50 miles 
it must be 4600°, or far higher than the fusing-point even of so stubborn 
a metal as platinum, which melts at 3080° Fahr.^ (b,) All over the world 
volcanoes exist from which steam and torrents of molten lava are from 
time to time erupted. Abundant as are the active volcanic vents, they 
form but a small proportion of the whole which have been in operation 
since early geological time. It has been inferred, therefore, that these 
numerous funnels of commimication with the heated interior could not 
have existed and poured forth such a vast amount of molten rock, unless 
they drew their supplies from an immense internal molten nucleus, (c.) 
When the products of volcanic action from different and widely-separated 
regions are compared and analysed, they are found to exhibit a remark- 
able uniformity of character. Lavas from Vesuvius, from Hecla, from 
the Andes, from Japan, and from New Zealand present such an agree- 
ment in essential particidars as, it is contended, can only be accounted 
for on the supposition that they have all emanated from one vast 
common source.*- (d.) The abundant earthquake -shocks which affect 
large areas of the globe are maintained to be inexplicable unless on the 
supposition of the existence of a thin and somewhat flexible crust. 
These arguments, it will be observed, are only of the nature of inferences 
drawn from observations of the present constitution of the globe. They 
are based on geological data, and have been frequently urged by geo- 
logists as supporting the only view of the nature of the earth's interior, 
supposed by them to be compatible with geological evidence. 

2. Th€ argument!^ in favour of the interned solidity of the earth are based 
on physical and astronomical considerations of the greatest importance. 
They may be arranged as follows : — 

((I.) Argument from precession and nutation. — The problem of the 
internal condition of the globe was attacked as far back as the year 1839 

* Bnt Lord Kelvin (Sir W. Tliomson) has shown that if the rate of increase of tempera- 
ture is taken to be 1* for every 51 feet for the first 100,000 feet, it will begin to diminish 
below that limit, being only 1* in 2550 feet at 800,000 feet, and then rapidly lessening. 
Trans. Roy. Soe. Edin. xxiii. p. 163. 

* See D. Forbes, Popular Science Review^ April, 1869. 



64 GEOGNOSY book ii 

by Hopkins, who calculated how far the planetary motions of precession and 
nutation would be influenced by the solidity or liquidity of the earth's 
interior. He found that the precessional and nutational movements 
could not possibly be as they are, if the planet consisted of a central core 
of molten rock surrounded with a crust of twenty or thirty miles in 
thickness ; that the least possible thickness of crust consistent with the 
existing movements was from 800 to 1000 miles; and that the whole 
might even be solid to the centre, with the exception of comparatively 
small vesicular spaces filled with melted rock.^ 

M. Delaunay * threw doubt on Hopkins' views, and suggested that, if 
the interior were a mass of suflBcient viscosity, it might behave as if it 
were a solid, and thus the phenomena of precession and nutation might not 
be aff'ected. Lord Kelvin (Sir W. Thomson), who had already arrived at 
the conclusion that the interior of the globe must be solid, and acquiesced 
generally in Hopkins' conclusions, remarked that the hypothesis of a 
viscous and quasi-rigid interior " breaks down when tested by a simple 
calculation of the amount of tangential force required to give to any 
globular portion of the interior mass the precessional and nutational 
motions which, with other physical astronomers, M. Delaunay attributes 
to the earth as a whole." ^ He held the earth's crust down to depths 
of hundreds of kilometres to be capable of resisting such a tangential 
stress (amounting to nearly y^**^ ^^ ^ gramme weight per square centi- 
metre) as would with great rapidity draw out of shape any plastic sub- 
stance which could properly be termed a viscous fluid, and he concluded 
" that the rigidity of the earth's interior substance could not be less than 
a millionth of the rigidity of glass without very sensibly augmenting the 
lunar nineteen-y early nutation. " * 

In Hopkins' hypothesis he assumed the crust to be infinitely rigid 
and unyielding, which is not true of any material substance. Lord 
Kelvin subsequently returning to the problem, in the light of his own 
researches in vortex-motion, found that, while the argument against a 
thin crust and vast liquid interior is still invincible, the phenomena of 
precession and nutation do not decisively settle the question of internal 
fluidity, as Hopkins, and others following him, had believed, though the 
solar semi-annual and limar fortnightly nutations absolutely disprove the 
existence of a thin rigid shell full of liquid. If the inner surface of the 
crust or shell were rigorously spherical, the interior mass of supposed 
liquid could experience no precessional or nutational influence, except in 
so far as, if heterogeneous in composition, it might suffer from external 
attraction due to non-sphericity of its surfaces of equal density. But 
" a very slight deviation of the inner surface of the shell from perfect 
sphericity would suffice, in virtue of the quasi -rigidity due to vortex- 

^ PhU. Trans. 1839, p. 381 ; 1840, p 193 ; 1842, p. 43 ; Brit, Assor. 1847. 

- In a paper on the hypothesis of the interior fluidity of the globe, CUymptes rendus, 
July 13, 1868. Oeol. Mag. v. p. 507. See also H. Hennessy, Compter rendus, 6 March, 
1871, Oeol. Mag. viii. p. 216. Nature, xv. p. 78. 0. Fisher, * Physics of the Earth's 
Crust,' 2nd Etlition, 1889. 

•'' Xature, February 1, 1872. •* Tjk. cCt. p. 258. 



56 GEOGNOSY book ii 

{('.) Argument from relative densities of melted and solid rock. — 
The two preceding arguments must be considered decisive against 
the hypothesis of a thin shell or crust covering a nucleus of molten 
matter. It has been further urged, as an objection to this hypothesis, 
that cold solid rock is more dense than hot melted rock, and that even 
if a thin crust were formed over the central molten globe it would immedi- 
ately break up and the fragments would sink towards the centre.^ Kecent 
experiments show that diabase (of density 3*017) contracts nearly 4 i)er 
cent on solidification, and that the resulting homogeneous glass has a density 
of only 2*717.- As has been already pointed out, the specific gravity of the 
interior is at least twice as much as that of the visible parts of the 
crust. If this difference be due, not merely to the effect of pressure, 
but to the presence in the interior of intensely heated metallic 8ul>- 
stances, we cannot suppose that solidified portions of such rocks as 
granite and the various lavas could ever have sunk into the centi-e of the 
earth, so as to build up there the honey-combed cavernous mass which 
might have served as a nucleus in the ultimate solidification of the 
whole planet. If the eai*liest formed portions of the comparatively 
light crust were denser than the underlying liquid, they would no doubt 
descend until they reached a stratum with specific gi*avity agreeing with 
their own, or until they were again melted.-* 

3. Ihjpothesis of a liquid mhsiraUnn between a solid nucleus and tht 
crust. — Since the early and natural belief in the liquidity of the earth's 
interior has been so weightily opposed by physical arguments, geologists 
have endeavoured to modify it in such a way as, if possible, to satisfy 
the requirements of physics, while at the same time providing an 
adequate explanation of the corrugation of the earth's crust, the 
phenomena of volcanoes, iVc* The hypothesis has })een proposed of "a 
rigid nucleus nearly approaching the size of the whole globe, covered by 
a fluid substratum of no great thickness, compared with the radius, upon 
which a crust of lesser density floats in a state of equilibrium." The 
nucleus is assumed to owe its solidity to "the enormous pressure of 
the superincumbent matter, while the crust owes its solidity to having 
become cool. The fluid substratum is not under sufficient pressiu-e to be 

^ Tliis objection lias been repeate^lly urged by Lord Kelvin. See Trans, Hoy. Soc. Edin, 
xxiii. p. 157 ; and Brit. Assoc. Hep. 1870, Sections, p. 7. 

* C. Barus, Pliii. Mafj. 1893, p. 174. It is nevertheless tnie that, from a cause merely 
mechanical, pieces of the original cold rock, though so much denser, will float for a time on 
the melted material. Ih. p. 189. 

^ See D. Forbes, Oed. Mag. vol. iv. p. 435. The evidence for the internal solidity of the 
earth is criticised by Dr. M. E. Wadaworth in the American Xaturaiistt 1884. 

■* See Dana in Siilinuins Journal^ iii. (1847), p. 147. Ainer. Journ. Science (1873). 
The hypothesis of a fluid substratum has been advocated by Shaler. Proc. BosL Nat, 
Hist. Soc. xi. (1868), p. 8. Geol. Mag, v. p. 511. J. Le Conte, Amei', Jonni, Set. 1872, 
1873. O. Fisher, Geol. Mag. v. (new series), pp. 291 and 551. ' Physics of the Earth's 
Crust,' 1883. [This author in his second edition modifies this view.] Hill, Oeiji. Mag. v. 
(new series), pp. 262, 479. The idea of a viscous layer between the solidifying central mass 
and the crust was present in Hopkins' mind. Brit. Assoc. 1848, Reports, ]>. 48. 



58 GEOGNOSY book ii 



M' 



% 4. Age of the Earth and Measures of Geological Time. — The 

age of our planet is a problem which may be attacked either from the 
geological or physical side. 

1. The geological arguments rest chiefly upon the observed rates 
at which geological changes are being effected at the present time, 
and is open to the obvious preliminary objection that it assumes 
the existing rate of change as the measure of past revolutions, — ^an 
assumption, however, which may be erroneous, for the present may be 
a period when all geological events march forward more slowly than 
they used to do. The argument proceeds on data partly of a physical 
and partly of an organic kind, (a.) The physical evidence is derived 
from such facts as the observed rates at which the surface of a 
country is lowered by rain and streams, and new sedimentary deposits 
are formed. These facts will be more particularly dwelt upon in 
later sections of this volume. If we assume that the land haJs been 
worn away, and that stratified deposits have l)een laid down, nearly 
at the same rate as at present, then we must admit that the stratified 
portion of the crust of the earth must represent a very vast period 
of time.^ (6.) On the other hand, human experience, so far as it 
goes, warrants the belief that changes in the organic world proceed 
>vith extreme slowness. Yet in the stratified rocks of the terrestrial 
crust, we have abundant proof that the whole fauna and flora of the 
earth's surface have passed through numerous cycles of revolution, — 
species, genera, families, orders, appearing and disappearing many 
times in succession. On any supposition, it must be admitted that 
these vicissitudes in the organic world can only have been effected 
with the lapse of vast periods of time, though no reliable standard 
seems to be available whereby these periods are to be measured. 
The argument from geological evidence indicates an interval of 
probably not much less than 100 million years since the earliest forms 
of life appeared upon the earth, and the oldest stratified rocks began to 
be laid down. 

2. The physical argument as to the age of our planet is based 
by Lord Kelvin upon three kinds of evidence: — (1) the internal heat 
and rate of cooling of the earth ; (2) the tidal retardatidn of the earth 
rotation ; and (3) the origin and age of the sun's heat. 

^ Dr. Croll put this j^eriod at not less, but possibly much more, than 60 million 
years. Dr. Haughton gives a much more extemled i>eriod. Rstimating the present 
rate of deposit of strata at 1 foot in 8616 years, assuming the former rate to have been 
ten times more rapid, or 1 foot in 861*6 years, and taking the thickness of the stratified 
rocks of the earth's crust at 177,200 feet, he obtains a minimum of 200,000,000 years 
for the whole duration of geological time : * Six Lectures on Physical Greography,* 1880, 
]). 94. Dr. Haughton has also proposed another geological measure of past time, 
based upon the assumed effects of continental upheaval {Proc, Roy. Soc. xxvi. (1877), 
1>. 534). But Professor Darwiu has shown it to be inadmissible. {Op, cit. xxrii. 
(1878), p. 179.) For various opinions regarding geological measures of time see J. Phillips, 
Hnt. Assoc, 1864 : C^oll,PAi7. Mag. 1868 : T. M'K. Hughes, Proc. Hoy. Inst. Great BHtain, 
March 24, 1876 : Dupont, Bull. Acad. Hoy. Belyique, viii. (1884) : T. Mellard Reade, Quart. 
Joum. Oeol. ,Soc. 1888, p. 291. 



PART I THE AGE OF THE EARTH 69 

(1.) Applying Fourier's theory of thermal conductivity, he pointed 
out as far back as the year 1862, that in the known rate of increase of 
temperature downward beneath the surface, and the rate of loss of heat 
from the earth, we have a limit to the antiquity of the planet. He 
showed, from the data available at the time, that the superficial 
consolidation of the globe could not have occurred less than 20 million 
years ago> or the underground heat would have been greater than it 
is ; nor more than 400 million years ago, otherwise the underground 
temperature would have shown no sensible increase downwards. He 
admitted that very wide limits were necessary. In subsequently discuss- 
ing the subject, he inclined rather towards the lower than the higher 
antiquity, but concluded that the limit, from a consideration of all the 
evidence, must be placed within some such period of past time as 100 
millions of years. He would now restrict the time to about 20 millions.^ 

(2.) The reasoning from tidal retardation proceeds on the admitted 
fact that, owing to the friction of the tide-wave, the rotation of the 
earth is retarded, and is therefore slower now than it must have been 
at one time. Lord Kelvin contends that had the globe become solid 
some 10,000 million years ago, or indeed any high antiquity beyond 100 
million years, the centrifugal force due to the more rapid rotation must 
have given the planet a very much greater polar flattening than it 
actually possesses. He admits, however, that though 100 million years 
a^ that force must have been about 3 per cent greater than now, yet 
" nothing we know regarding the figure of the earth and the disposition of 
land and water would justify us in saying that a body consolidated when 
there was more centrifugal force by 3 per cent than now, might not now 
be in all respects like the earth, so far as we know it at present." - 

(3.) The third kind of evidence leads to results similar to those derived 
from the two previous lines of reasoning. It is based upon calculations 
as to the amount of heat that would be available by the falling together 
of masses from space, which gave rise by their impact to our sun, and 
the rate at which this heat has been radiated. Assuming that the sun 
has been cooling at a uniform rate. Professor Tait concludes that it cannot 
have supplied the earth, even at the present rate, for more than about 15 
or 20 million years.^ Lord Kelvin also believes that the sun's light will 
not last more than 5 or 6 millions of years longer.* 

There can be no doubt that the demands of the earlier geologists for 
an unlimited duration of past time, for the accomplishment of geological 
history, were extravagant and unnecessary. But it may be questioned 
how far the recent limitation of time proposed from physical consider- 

* Trans. Roy. Soc Edin. xxiii. p. 157. Trans. Qeol. Soc. Glasgow, iii. p. 25. 'Popular 
Lectures and Addresses,' 2nd etlit. (1891), p. 397. Professor Tait reduces the period to 10 
or 15 mUlions. 'Recent Advances in Physical Science,' p. 167. 

' Trans. Geol. Soc. Glasgow, iii. p. 16. Professor Tait, in repeating this argument 
concludes that, taken in connection with the previous one, ** it probably reduces the possible 
period which can be allowetl to geologists to something less than 10 millions of years." 
'Recent Advances,' p. 174. Compare Newcomb, 'Popular Astronomy,* p. 505. 

» Op. cit. p. 174. * 'Popular Lectures, etc.,' p. 397. 



6 GEOGNOS Y book ii 

ations are really founded on well-established facts. The argument from 
the geological record in favour of a much longer period than physicists 
are disposed to concede is so strong that one is inclined to believe that 
these writers have overstated their case. The evidence from the nature 
of the sedimentary rocks, and from the succession of organic remains in 
these rocks, appears to me to demand an amount of time not far short of 
the hundred millions of years originally granted by Lord Kelvin.^ 



Part II. — Ax Account of thk Composition of the PIarth's 

Crust — Minerals and Kocks. 

The earth's crust is composed of minei^l matter in various aggregates 
included under the general term Rock. A rock may be defined as a 
mass of matter composed of one or more simple minerals, having 
usually a variable chemical composition, with no necessaiily symmetrical 
external form, and ranging in cohesion from mere loose debris up to 
the most compact stone. Granite, lava, sandstone, limestone, gi*avel, sand, 
mud, soil, marl and peat, are all recognised in a geological sense as 
rocks. The study of rocks is known jis Lithology, Petrogiuphy or 
Petrology. 

It will be most convenient to treat — Ist^ of the general chemical 
constitution of the crust ; 2nd, of the minerals of which rocks mainly 
consist ; 3rd, of the methods employed for the determination of rocks ; 
4th, of the external characters of rocks, 5th, of the internal texture and 
structure of rocks ; 6th, of the classification of rocks ; and 7th, of the 
more important rocks occurring as constituents of the earth's crust. 



§ i. (General Chemical Constitution of the Crust. 

Direct acquaintance with the chemical constitution of the globe must 
obviously be limited to that of the crust, though by inference we may 
eventually reach highly probable conclusions regarding the constitution 
of the interior. Chemical research has discovered that some sixty-four ^ 
simple or as yet undecomposable bodies, called elements, in various pro- 
portions and compounds, constitute the accessible i^art of the cnist. Of 
these, however, the great majority are comparatively of rare occurrence. 
The crust, so far as we can examine it, is mainly ])uilt up of about sixteen 
elements, which may be arranged in the two following groups, the most 
abundant bodies being placed first in each list : — 

^ I have touched on this question in my Presidential Addre.^s to the British Association 
1892. But see a i^aper by Mr. Clarence King, Ain^r. Journ, Sci. xlv. (1893). 

- This number has within the last few years been increased by the allege*! discovery of 
no fewer than fourteen new metals. Some of the.se Ixxlies, however, have not yet been 
satisfactorily proved to be new. T. S. Humpidge, yatitre^ xxii. p. 232. 



62 GEOGNOSY book u 

magmas co- existing in the earth's crust, the one beneath the other, 
according to their specific gravities. The upper or outer shell, which he 
termed the acid or siliceous magma, contains an excess of silica, and has 
a mean density of 2*65. The lower or inner shell, which he called the 
basic magma, has from six to eight times more of the earthy bases and 
iron-oxides, with a mean density of 2*96. To the former he assigned 
the early plutonic rocks, granite, felsite, &c., with the more recent 
trachytes ; to the latter he relegated all the heavy lavas, basalts, diorites, 
v^c. The ratio of silica is 7 in the acid magma to 5 in the basic 
Though the proportion of silicic acid or of the earthy and metallic bases 
cannot be regarded as any certain evidence of the geological date of 
rocks, nor of their probable depth of origin, it is nevertheless a fact that 
(with many important exceptions) the eruptive rocks of the older geo- 
logical periods are very generally super-silicated and of lower specific 
gravity, while those of later time are very frequently poor in silica, but 
rich in the earthy bases and in iron and manganese, with a consequent 
higher specific gravity. The latter, according to Durocher, have been 
forced up from a lower zone through the lighter siliceous crust The 
sequence of volcanic rocks, as first announced by Eichthofen, has an 
interesting connection with this speculation.^ 

The main mass of the earth's crust is composed of a few predominant 
compounds. Of these in every respect the most abundant and important 
is Silicon-dioxide or Silica (Kieselerde) SiO^. As the fundamental in- 
gredient of the mineral kingdom, it forms more than one half of the 
known crust, which it seems to bind firmly together, entering as a main 
ingredient into the composition of most crystalline and fragmental rocks 
as well as into the veins that traverse them. It occurs in the free state 
as the abundant rock -forming mineral quartz, which strongly resists 
ordinary decay, and is therefore a marked constituent of many of the 
more enduring kinds of rock. As one of the acid-forming oxides (H^SiO^, 
Silicic acid, Kieselsaure) it forms combinations with alkaline, earthy, and 
metallic bases, which appear as the prolific and universally diffused 
family of the silicates. Moreover, it is present in solution in terrestrial 
and oceanic waters, from which it is deposited in pores and fissures of 
rocks. It is likewise secreted from these waters by abundantly diffused 
species of plants and animals (diatoms, radiolarians, &c.) It has been 
largely eftective in replacing the organic textures of former organisms, 
and thus preserving them as fossils. 

Alumina or aluminium-oxide (Thonerde), AlyO^, occurs sparingly as 
corundum, which, however, according to F. A. Genth, was the original 
condition of many now abundant complex aluminous minerals and rocks. 
The most common condition of aluminium is in union with silica. In 
this form it constitutes the basis of the vast family of the aluminous 
silicates, of which so large a portion of the crystalline and fragmental 
rocks consists. Exposed to the atmosphere, these silicates lose some of 
their more soluble ingredients, and the remainder forms an earth or clay 
consisting chiefly of silicate of aluminium. 

* Postea, Book III. Part I. Section i. § 6. 



PART n § i COMPOSITION OF THE CRUST 63 

Carbon is the fundamental element of organic life. In combination 
with hydrogen, as well as with oxygen, nitrogen and sulphur, it forms 
the various kinds of coal, and thus takes rank as an important rock- 
forming element As carbon-dioxide, CO,^ it is present in the air, in 
rain, in the sea and in ordinary terrestrial waters. This oxide is soluble 
in water,^ giving rise then to a dibasic acid termed Carbonic Acid 
(Kohlensaure), CO(OH)o or HgCOg, which forms carbonates, its combina- 
tion with calcium having been instrumental in the formation of vast 
masses of solid rock. Carbon-dioxide constitutes a fifth part of the 
weight of ordinary limestone. 

Sulphur (Soufre, Schwefel) occurs uncombined in occasional deposits 
like those of Sicily and Naples, to be afterwards described, also in union 
with iron and other metals as sulphides ; but its principal condition 
as a rock -builder is in combination with oxygen as sulphuric acid 
(Schwefelsiiure) H2S0^ which forms sulphates of lime, magnesia, &c. 

Calcium enters into the composition of many crystalline rocks in 
combination with silica and with other silicates. But its most abundant 
form is in union with carbon-dioxide, when it ap|)ears as the mineral, 
calcite (CaCOg), or the rock, limestone. Calcium -carbonate, being 
soluble in water containing carbonic acid, is one of the most universally 
diffused mineral ingredients of natural waters. It supplies the varied 
tribes of mollusks, corals, and many other invertebrates with mineral 
substance for the secretion of their tests and skeletons. Such too has 
been its office from remote geological periods, as is shown by the vast 
masses of organically-formed limestone, which enter so conspicuously into 
the structure of the continents. In combination with sulphuric acid, 
calcium forms important beds of gypsum and anhydrite. 

Magnesium, Potassium, and Sodium play a less conspicuous but still 
essential part in the composition of the earth's crust. Magnesium, in 
combination with silica, forms a class of silicates of prime importance in 
the composition of volcanic and metamorphic rocks. As a carbonate, it 
unites with calcium-carbonate to form the widely diftused rock, dolomite. 
In union with chlorine, it takes a prominent place among the salts of sea- 
water. Potassium or Sodium, combined with silica, is present in small 
quantity in most silicates. In union with chlorine, as common salt, 
sodium is the most important mineral ingredient of sea-water, and can be 
detected in minute quantities in air, rain, and in terrestrial waters. In 
the old chemical formulae hitherto employed in mineralogy the metals of 
the alkalies and alkaline earths are represented as oxides. Thus lime 
(calcium-monoxide), soda (sodium-monoxide), potash (ix)tassium-monoxide), 
magnesia (magnesium-oxide), are denoted as in union with carbonic acid, 
sulphuric acid, silica, &c., forming carbonates, sulphates, silicates of lime, 
soda, &c. 

Iron and Manganese are the two most common heavy metals, occurring 
both in the form of ores, and as constituents of rocks. Iron is the great 
pigment of natm*e. Its peroxide or sesquioxide, now known as ferric 

* One volume of water at 0* C. dissolves 1*7967 volumes of carbon-dioxide ; at 15" C 
the amount is reduced to 1*0020 volumes. 



64 GEOrrNOSY wmjh ii 



oxide, forms large ininera] masses, and together with the protoxide or 
ferrous oxide, occurs in smaller or larger proportions in the great majority 
of crystalline rocks. Iron (as sulphate or in combination with organic 
acids) is removed in solution in the water of springs, and precipitated as 
a hydrous |)eroxide. Manganese is commonly associate<l with iron in 
minute pro}>ortion8 in igneous i*ocks, and being similarly removed in 
solution in water, is thrown down as bog manganese or wad. 

Silicic Acid, Carbonic Acid, and Sulphuric Acid are the three acids 
with which most of the bases that compose the earth's crust have been 
combined. With these we may connect the water which, besides merely 
percolating through rocks, or existing enclosed in the vesicles of minerals, 
hsis been chemically absorbed in the process of hydration, and which thus 
constitutes more than 10 or even 20 per cent of some rocks (gypsum). 

Chemical analysis has revealed the numerous combinations in which 
the elements are united to form minerals and rocks. Considerable 
additional light has Ixjen thrown on the subject by chemical synthesis, 
that is, by artificially producing the minerals and rocks which are found 
in nature. The experiments have been varied indefinitely so as to 
imitate as far as jwssible the natural conditions of production. Further 
reference to this subject will be found on pp. 89, 297 et seq. 

Although every mineral may be made to yield data of more or less 
geological significiince, only those minerals need be referred to here which 
enter as chief ingredients into the com|)osition of rock-masses, or which 
are of frequent occurrence as accessories, and special note may be taken 
of those of their characters which are of main interest from a geological 
point of view, such as their modes of occiu*rence in relation to the 
genesis of rocks, and their weathering as indicative of the nature of 
rock-decom})osition. 

§ ii. Bock-forminff Minerals. 

Minerals, as constituents of rocks, occur in four conditions, according 
to the circumstances under which they have been produced. 

(1.) Crystalline, as {a) more or less regularly defined crystals, which, 
exhibiting the outlines proper to the mineral to which they belong, are 
said to l)e uliomoiyhk ; {h) amorphous granules, aggi-egations or crystalloids, 
having an internal crystalline structure, in most cases eiisily recognisable 
with polarized light, as in the quartz of granite, and an external form 
which has })een determined by contact with the adjacent mineral particles ; 
such crystiilline Ixxlies which do not exhibit their proper crystalline outlines 
are said to be aUotrwnuyrphic ; (c) " crystallites " or " microlites," incipient 
forms of crystiilliziition, which are described on p. 115. The crystalline 
condition may arise from igneous fusion, aqueous solution, or sublimation.^ 

(2.) Glaniiy or vitremis^ as a natural glass, usually including either crystals 
or crystallites, or both. Minerals have assumed this condition from a 
state of fusion, also from solution. The glass may consist of several 
minerals fused into one homogeneous substance. Where it has assumed a 

^ For the microscopic characters of minerals auil rockN, see p. lOS. 



66 GEOGNOSY book n 

stone have formed part of the rock ever since it was accumulated, and 
are its essential constituents. Yet each of these once formed part of some 
older rock, the destruction of which yielded materials for the production 
of the sandstone. The minute crystals of zircon, rutile, tourmaline and 
other minerals so often found in sands, clays, sandstones, shales and other 
sedimentary deposits, have been derived from the degradation of older 
crystalline rocks. 

The same mineral may occur both as an original and as a secondary 
constituent Quartz, for example, appears everywhere in both conditions ; 
indeed, it may sometimes be found in a twofold form even in the same 
rock, though there is then usually some difference between the original 
and secondary quartz. A quartz-felsite, for instance, aboimds in original 
little kernels, or in double pyramids of the mineral, often enclosing fluid 
cavities, while the secondary or accidental forms usually occur in veins, 
reticulations, or other irregular aggregates. 

Accessory minerals frequently occur in cavities where they have 
had some room to crystallize out from the general mass. The " drusy " 
cavities, or open spaces lined with well developed crystals, found in some 
granites are good examples, for it is there that the non-essential minerals 
are chiefly to be recognised. The veins of segi-egation found in many 
crystalline rocks, particularly in those of the granite series, are further 
illustrations of the original separation of mineral ingredients from the 
general magma of a rock (see pp. 578, 580). 

In some cases minerals assume a concretionary shape, which may be 
observed chiefly though not entirely in rocks formed in water. Some 
minerals are particularly prone to occur in concretions. Siderite (ferrouA 
carbonate) is to be found in abundant nodules, mixed with clay and 
organic matter among consolidated muddy deposits. Calcite (caJcium- 
carbonate) is likewise abundantly concretionary. Silica in the forms of 
chert and flint appears in irregular concretions, in calcareous formations^ 
composed mainly of the remains of marine organisms. 

Secondary minerals have been developed as the result of subsequent 
changes in rocks, and are almost invariably due to the chemical action of 
percolating water, either from above or from below. Occurring under 
circumstances in which such water could act with eff^ect, they are found 
in cracks, joints, Assures and other divisional planes and cavities of rocks^- 
especially in the minute interspaces between the component grains or 
minerals. Subterranean channels, frequently several feet or even 
yards wide, have been gradually filled up by the deposit of mineral 
matter on their sides (see the Section on Mineral Veins). The cavitiea 
formed by expanding steam in ancient lavas (amygdaloids) have oflered 
abundant opportunities for deposits of this kind, and have accordingly 
been in large measure occupied by secondary minerals (amygdales), as 
calcite, chalcedony, quartz and zeolites. 

In the subjoined list of the more important rock-forming minerals, 
attention is drawn mainly to those features that are of geological 
importance ; the physical, chemical and microscopic characters of these 
minerals will be found in a text-book of mineralogy or petrography. 



PART n § ii ^ ROCK-FORMING MINERALS 67 



Reference is therefore made here to features of more special signifi- 
cance to the geologist, such as modes of occurrence, whether original 
or secondary ; modes of origin, whether igneous, aqueous, or organic ; 
pseudomorphs, that is, the various minerals which any given mineral has 
replaced, while retaining their external forms, and likewise those which 
are found to have supplanted the mineral in question while in the same 
way retaining its form — a valuable clue to the internal chemical changes 
which rocks undergo from the action of percolating water (Book III. 
Part IL Section ii. §§ 1 and 2) ; and lastly, characteristics or peculiarities 
of weathering, where any such exist that deserve special mention. 

1. Nathte elements are comparatively of rare occurrence, and only two of them, 
Carbon and Sulphur, occasionally play the part of noteworthy essential and accessory 
eonstitaents of rocks. A few of the native metals, more especially copper and gold, now 
and then appear in sufficient quantity to constitute commercially important ingredients 
of veins and rock>masses. 

Qraphiie is found chiefly in ancient crystalline rocks, as gneiss, mica-schist, granite, 
kc, ; some of the Laurentian limestones of Canada being so full of the diffused mineral 
as to be profitably worked for it ; in rare instances coal has been observed changed into 
it by intrusive basalt (Ayrshire). In some cases graphite results from the alteration of 
imbedded organic matter, especially remains of plants ; but its presence, and that of 
diamond, among ancient crystalline rocks and in meteorites can hardly be thus 
acooonted for. Occasionally it is observed as a |)seudomorph after calcite and pyrites, 
and sometimes enclosing sphene and other minerals.^ 

Sulphur occurs 1st, as a product of volcanic action in the vents and fissures of active 
and dormant cones. Volcanic sulphur is formed from the oxidation of the sulphuretted 
hydrogen, so copiously emitted with the steam that issues from volcanic vents, as at the 
Soliatara, near Naples. It may also be produced by the mutual decomposition of the 
sune gas and anhydrous sulphuric acid. 2nd, in beds and layers, or diff'used particles, 
resulting from the alteration of previous minerals, |>articularly sulphates, or from deposit 
in water through decom|)osition of sulphuretted hydrogen. The frequent crystallization 
of sulphur shows that the mineral must have been formed at ordinary temi>eratures, for 
its natural crystals melt at 238*1*' Fahr. Its formation may be observed in progress at 
many sulphureous springs, ¥^ere it falls to the bottom as a i^ale mud through the 
oxidation of the sulphuretted hydrogen in the water. It occurs in Sicily, Spain and 
elsewhere, in beds of bituminous limestone and gypsum. These strata, sometimes full 
of remains of fresh-water shells and plants, are interlaminated with sulphur, the very 
shells being not infrequently replaced by this mineral. Here the presence of the sulphur 
may be traced to the reduction of the calcium-sulphate to the state of sulphide, through 
the action of the decomposing organic matter, and the subsequent production and 
decomposition of sulphuretted hydrogen, with consequent liberation of sulphur.^ The 
sulphur deposits of Sicily furnish an excellent illustration of the alternate deposit of 
snlphor and limestone. They consist mainly of a marly limestone, through which the 
sulphur is partly disseminated and partly interstratified in thin laminse and thicker 
layers, some of which are occasionally 28 feet deep. Below these deposits lie older 
Tertiary gypseous formations, the decomi)osition of which has probably produced the 
deposits of sulphur in the overlying more recent lake basins.' The weathering of sulphur 



* Vom Rath. Sitzungsber. Wien, Akad. x. p. 67 ; Sullivan in Jukes' * Manual of 
Geology,' 3rd edit (1872), p. 56. 

' Braun, Bull. Soe. Oiol, France^ Ist ser. xii. p. 171. 
» Memarie del R, Comitalo Geologico d' Italia, i. (1871). 



68 GEOGNOSY book ii 

is exemplified on a considerable scale at these Sicilian deposits. The mineral, in 
presence of limestone, oxygen and moisture, becomes sulphuric acid, which, combining 
with the limestone, forms gyjisum, a curious return to what was probably the original 
substance from the decom|)osition of which the sulphur was derived. Hence the site of 
the outcrop of the sul])hur beds is marked at the surface by a white earthy rock, or 
borscale, which is regarded by the miners in Sicily to be a sure indication of snlphur 
underneath, as the gossan of Cornwall is indicative of underlying metalliferous veins.* 

Iron, the most imjtortant of all the metals, is found only sparingly in the native 
state, in blocks which have fallen as meteorites, also in grains or dust enclosed in 
hailstones, in snow of the Alps, Sweden and Siberia, in the mud of the ocean floor at 
remote distances from land, and in some eruptive rocks. Tliere can be no doubt that a 
small but constant supply of native iron (cosmic dust) is falling upon the earth's sur£M)e 
from outside the terrestrial atmosphere.^ This iron is alloyed with nickel, and contains 
small quantities of cobalt, copper and other ingredients. Dr. Andrews, however, 
showed in 1852 that native iron, in minute spicules or granules, exists in some basalts 
and other volcanic rocks, ^ and Mr. J. Y. Buchanan has detected it in appreciable 
quantity in the gabbro of the west of Scotland. It occurs also in the basalts of Bohemia 
and Greenland.** 

In the great majority of eases the Oxides occur combined with some acid. A few 
uncombined take a prominent place as essential constituents or frequent ingredients of 
rocks, especially the oxides of silicon and iron. 

2. Silica (SiOa) is found in three chief forms, Quartz, Tridymite, and Opal. 

Quartz is abundant as (1) an essential constituent of rocks, as in granite, gneiss, 
mica-schist, rhyolite (quartz-trachyt*,) quartz-poq)hyry, sandstone ; (2) a secondaiy 
ingredient, wholly or ])artially tilling veins, joints, cracks, and cavities. It has been 



* Jotirn. Soc. Art^j 1873, p. 170. E. Lcdoux, Ann. dcs MineSy 7™* s<Jr. vii. p. 1. The 
Sicilian sulphur beds belong to the Oeningen stage of the Upper Tertiary deposits. They 
coutain numerous plants and some iusects. H. T. Geyler, PulaonUtffraphica, xxiii., Zi-c^. 
9, p. 317. Von Lasaulx, Xeties Jahrb, 1879, p. 490. 

- See Ehrenl^erg, Frorieps Xotizenj Feb. 1846 ; Nordenskiold, Comptes renduSj IxxviL 
1». 463, Ixxviii. p. 236. Tissandier, op. cit. Ixxriii. p. 821, Ixxx. p. 58, Ixxxi. p. 676. See 
Ixxv. (1872) p. 683. Yung, Ball, Soc. Vaudoise Set. Nat. (1876), xiv. p. 493. Ranyard, 
Monthly Not. Roy. Astron. Soc xxxix. (1879) p. 161. T. L. Phipson, Comptes rend. 
Ixxxiii. p. 364. A Committee of the British Association was appointed in 1880'to investi- 
gate the subject of cosmic dust. See its reports for 1881-83. 

•* Brit. Assoc. Rep. 1852, postea, p. 457. 

* Nordenskiold describes fifteen blocks of iron on the island of Disco, Greenland, the 
weight of the two largest })eiug 21,000 and 8000 kilogrammes (20 and 8 tons, respectively). 
He observed that at the same locality, the underlying basalt contains lenticular and disc- 
shax>ed blocks of precisely similar iron, and inferred that the whole of the blocks may belong 
to a meteoric shower which fell during the time (Tertiary) when the basalt was poured ont 
at the surface. He dismissed the suggestion that the iron could possibly be of tellnric 
origin {Ocol. Mag. ix. (1872) p. 462). But the microscope reveals in this basalt the 
presence of minute particles of native iron which, associated with viridite, ore moulded round 
the crystals of labradorite and augite (Fouquc and Michel-Levy, op. cit. p. 443). Steen- 
strup, Daubr^e, and others appear therefore to be justified in regarding this iron as derived 
from the inner metallic portions of the globe, which lie at depths inaccessible to our 
ob8t|r\'ations, but from which the vast Greenland basalt eruptions have brought up traces to 
the surface (K. J. T. Steenstrup, Vid. Medd. Nat. Foren. Copenhagen (1875) No. 16-19, 
p. 284 ; Zeitsch. Deutsch. Ged. Ges. xxriii. (1876) p. 225 ; Mineralog. Mag. July, 1884. 
F. Wohler, Neties Jahrb. 1879, p. 832. Daubree, Discours Acad. Set. 1 March 1880, p. 
17. W. Flight, Geol. Mag. ii. (2nd ser.) p. 152. 



PART II § ii ROCK-FORMING MINERALS 69 



produced from (a) igneous action, as in volcanic rocks ; {b) aquo-igneous or plutonic 
action, as in granites, gneisses, &c. ; (c) solution in water, as where it lines cavities or 
replaces other minerals. The last mode of formation is that of the crystallized quartz 
and chalcedony found as secondary ingredients in rocks. 

The study of the endomorphs and pseudomorphs of quartz is of great importance in 
the investigation of the history of rocks. No mineral is so conspicuous for the variety 
of other minerals enclosed within it. In some secondary quartz-crystals, each prism 
forms a small mineralogical cabinet enclosing a dozen or more distinct minerals, as 
mtile, haematite, limonite, pyrites, chlorite, and many others.^ Quartz may be 
obserred replacing calcite, aragonite, siderite, gypsum, rock-salt, haematite, &c. This 
facility of replacement nuikes silica one of the most valuable petrifying agents in nature. 
Organic bodies which have been silicified retain, often with the utmost i)erfection, their 
minutest and most delicate structures. 

Quartz may usually be identified by its external character, and esjiecially by its 
vitreous lustre and hardness. When in the form of minute blebs or crystals, it may 1)e 
recognised in many rocks with a good lens. Under the microscope, it presents a 
characteristic brilliant chromatic jiolarization, and in convergent light gives a black 
cross. Where it is an original and essential constituent of a rock, quartz very commonly 
contains minute rounded or irregular cavities or pores, jtartially filled with liquid. So 
minute are these cavities that a thousand millions of them may, when they are closely 
aggregated, lie within a cubic inch. Tlie liquid is chiefly water, not uncommonly 
containing sodium chloride or other salt, sometimes liquid carbon -dioxide and hydro- 
carbons.' Chalcedony exhibits under the microscope a minute radial fibrous structure. 

Rock-crystal and crystalline quartz resist atmospheric weathering with great per- 
sistence. Hence the quartz-grains may usually be easily discovered in the weathered 
cmst of a quartziferous igneous rock. But corroded quartz-crystals have been observed 
in exposed mountainous situations, with their edges rounded and eaten away.' The 
chalcedonic and more or less soluble forms of silica are more easily affected. Flint and 
many forms of coloured chalcedony weather with a white crust. But it is chiefly from 
the weathering of silicates (especially through the action of organic acids) that the 
soluble silica of natural waters is derived. (Book III. Part II. Section ii. § 7). 

Ttidyxnite has been met with chiefly among volcanic rocks (trachytes, andesiten, 
kc ), both as an abundant constituent of those which have been poured out in the form 
of lava, and also in ejected blocks (Vesuvius).'* 

Opal, a hydrous condition of silica formed from solution ifi water, is usually 
disseminated in veins and nests through rocks. Semi-opal occasionally replaces the 
original substance of fossil wood (wood-opal). Several forms of o^^l are deposited by 
geysers, and are known under the general appellation of sinters. Closely allied to the 
oi«l8 are the forms in which hydrous (soluble) silica appears in the organic world, where 
it constitutes the frustules of diatoms, the skeletons of radiolaria, &c. Tiipoli powder 
(Kieselgnhr), randanite, and other similar earths, are composed mainly or wholly of the 
remains of diatoms, kc. 

Oomndiim, aluminium- oxide, is found in crystalline rocks, particularly in certain 
serpentines and schists, gneiss, granite, dolomite, and rocks of the metamorphic series. 

8. Iron Oxides. — Four minerals, coniiK>8ed mainly of iron oxides, occur abundantly 

* See Sullivan, in Jukes' 'Manual of Geologj-,' 3rd edit. (1872), p. 61. 

' See. Brewster, Trans, Roy. Soc, Edin. x. p. 1. Sorby, Quart. Joum. Ged. Soc. xiv. 
p. 453. Proc Roy. Sac. xv. p. 153 ; xvii. p. 299. Zirkel, * Mikroskopische Beschaffenheit 
der Mineralien und Oesteine,' p. 39. Rosenbusch, ' Mikroskopische Physiographie,' i. ]>. 30. 
Hartley, Joum. Ckem. Sac. February, 1876. The occurrence of fluid-cavities in the crystals 
of rocks is more fully described in Part II. § iv. of this Book. 

' Roth, Ckem. Ged. L p. 94. 

* Vom Rath, Z. DeuUch. OeoL. Ges. xxv. p. 236, 1873. 



70 GEOGNOSY book n 

as essential and accessory ingredients of rocks. Haematite, Limonite, Magnetite, and 
Titanic iron. 

Hnmatite (Fer oligiste, Rotheisen, Eisenglanz, Fe3O3=Fe70, O30) in the crystal- 
lized form occurs in veins, as well as lining cavities and fissures of rocks. The 
fibrous and more conmion form (which often has portions of its mass passing into the 
crystallizecl condition) lies likewise in strings or veins ; also in cavities, which, when of 
large size, have given opportunity for the deposit of great masses of hfematite, as in 
cavernous limestones (Westmoreland). It occurs with other ores aifd minerals as an 
abundant component of mineral veins, likewise in beds interstratified with sedimentaxy 
or schistose rocks. Scales and specks of opaque or clear bright red haematite, of frequent 
occurrence in the crystals of rocks, give them a reddish colour or i)eculiar lustre (perthite, 
stilbite). Haematite appears abundantly as a product of sublimation in clefts of volcanic 
cones and lava streams. It is probably in most cases a deposition from water, resulting 
from the alteration of some previous soluble combination of the metal, such as the 
oxidation of the sulphate, and occurs in veins and beds, and as the earthy pigment that 
gives a red colour to sandstones, clays and other rocks. It is found pseudomorphons 
after ferrous carltonate, and this has pro1)ably been the origin of beds of red ochre occa- 
sionally intercalated among stratified rocks. It likewise replaces calcite, dolomite, 
quartz, barytes, pyrites, magnetite, rock-salt, fluor-spar, &c. 

Limonite (BroiiTi iron-ore, 2FesO, -f- SH^O = Fe203 85-56, H2O 14-44) occurs in beds 
among stratified formations, and may be seen in the course of deposit, through the action 
of organic acids, on marsh-land (l>og-iron-ore) and lake-bottoms. (Book IV. Part XL 
Section iiL ) In the form of yellow ochre, it is precipitated from the waters of chalybeate 
springs containing green vitriol derived from the oxidation of iron -sulphides.^ It is a 
common decomposition product in rocks containing iron among their constituents. It is 
thus always a secondary or derivative substance, resulting from chemical alteration. It 
is the usual pigment which gives tints of yellow, orange and broiMi to rocks. The 
{iseudomorphous forms of limonite show to what a large extent combinations of iron aie 
carried in solution through rocks. The mineral has been foimd replacing calcite, siderite, 
dolomite, haematite, magnetite, pyrite, niarcasite, galena, blende, gypsum, barytes, 
fluor-sj>ar, pyroxene, quartz, garnet, beryl, &c. 

Magnetite (Fer oxydule, Magneteisen, FezOi) occurs abundantly in some schists, in 
scattered octohedral crystals ; in crystalline massive rocks like granite, in diffused grains 
or minute crystals ; among some schists and gneisses (Non\'ay and the eastern states of 
North America), in massive beds ; in basalt and other volcanic rocks, as an essential 
constituent, in minute octohedral crystals, or in granules or crystallites. It is likewise 
found as a pscudomorphous secondary pro<luct, resulting from the alteration of some 
previous mineral, as olivine, haematite, j)yrite, quartz, hornblende, augite, garnet and 
sj>hpnc. It occurs AN-ith hapmatite, Ac, as a product of sublimation at volcanic foci, where 
chlorides of the metals in presence of st«am are resolved into hydrochloric acid and 
anhydrous oxides. It may thus result from either wpieous or igneous operations. It 
is liable to weather by the reducing effects of <lecomi)08ing organic matter, whereby it 
heconies a carbonate, and then by ex}K)sure jwusses into the hydrous or anhydrous 
I»eroxide. Tlie magnetite grains of basalt-rocks are very generally oxidized at the 
surface, and sometimes even for some depth inwanl. 

Titanic Iron (Titanifcrous Iron, !Meuaccanit(^ Ilmenite, Fer titan^, Titaneisen 
(FeTi)203) occurs in scattered grains, ]>lates and crystals as an abundant constituent of 
many cr^'stalline ixxiks (l>asalt-rocks, diabase, gabbro and other igneous masses) ; also in 
veins or l)e<ls in syenite, serjKmtine and metamorphic rocks ; ^ scarcely to be distinguished 
from magnetite when seen in small jjaiticles under the mi(?roscoj)e, but i)osses8ing a 

' Sullivan, Jukes' 'Manual of Geology,' p. 63. 

"^ Some of the Canadian mosses of this mineral are 90 feet thick and many yards in 
length. 



PART II § ii ROCK-FORMING MINERALS 71 

brown aemi-metallic lustre with reflected light ; resists corrosion by acids when the 
powder of a rock containing it is exposed to their action, while magnetite is attacked 
and diBsolyed. Titanic iron frequently resists weathering, so that its black glossy 
granules project from a weathered surface of rock. In other cases, it is decomposed 
either by oxidation of its protoxide, when the usual brown or yellowish colour of the 
hydrous ferric oxide appears, or by removal of the iron. The latter is believed to be 
the origin of a peculiar milky white opaque substance, frequently to be observed under 
the microscope, surrounding and even replacing crystals of titanic iron, and named 
Leocoxene byGiimbeL^ In other cases the decomposition has resulted in the production 
of sphene. 

Cbromite (FeCrjOi) occurs in black opaque grains and crystals not infrequently in 
altered olivine-rocks. 

SpiiiAU, a group of minerals, may be taken here. Tliey are closely related to each 
other, having cubic forms and varying in composition from magnetite (see above) at the 
one end to spinel (}igA\fi^) at the other. They are not infrequent as minute grains or 
crystals in some igneous and metamorphic rocks. Between magnetite and spinel come 
intermediate varieties, as chroniUe (see above), Picotite^ Hercynite and Pleonaste. 

4. Manganese Oxides are frequently associated with those of iron in ordinary rock- 
forming minerals, but in such minute proportions as to have been generally neglected in 
analyses. Their presence in the rocks of a district is sometimes show^ by deposits of 
the hydrous oxide in the forms of Psilomelane (H2Mn04+H20), and Wad (Mn03+ 
MnO + H3O). These deposits sometimes take place as black or dark brown branching, 
plant-like or dendritic impressions between the divisional planes of close-grained rocks 
(limestone, felsite, &c.), sometimes as accumulations of a black or brown earthy substance 
in hollows of rocks, occasionally as deposits in mai'shy places, like those of bog-iron-ore, 
and abundantly on some parts of the sea-floor. (See p. 458.) 

5. Silicates. — These embrace by far the largest and most imiwrtant series of rock- 
forming minerals. Their chief groups are the anhydrous aluminous and magnesian 
silicates embracing the Felspars, Hornblendes, Augites, Micas, &c., and the Iiydrous 
silicates which include the Zeolites, Clays, talc, chlorite, ser])entiue, &e. 

The family of the Felspars forms one of the most important of all the constituents 
of rocks, seeing that its members constitute by much the largest portion of the plutonic 
and volcanic rocks, are abundantly present among many crystalline schists, and by their 
decay have supplied a gi-eat i>art of the clay out of which argillaceous sedimentary forma- 
tions have been constructed. 

The felspars are usually divided into two series. 1st, The orthoclastic or monoclinic 
felspars, consisting of two sjtecies or varieties, Orthoclase and Sanidine ; and 2nd, The 
pUgioclastic or triclinic felspars, among which, as constituents of rocks, may be men- 
tioned the species albite, anorthite, oligoclase, andesine, labradorite, and microcline. 

OrthocUM (K3O 16-89, ALjO, 18-43, SiOa 64-68) occurs abundantly as an original 
constitnent of many crystalline r«cks (granite, syenite, felsite, gneiss, &c.), likewise in 
cavitiefi and veinings in which it has segregatefl from the surrounding mass (pegmatite) ; 
seldom found in unaltered sedimentary rocks except in fragments derived from old 
crystalline masses ; generally associated with quartz, and often Mith hornblende, while 
the felspars less rich in silica more rarely accomjtany free quartz. It is an original con- 
stitnent of plutonic and old volcanic rocks (granite, felsite, &c.), and of gneiss and various 
schists. A few examples have been noticed where it has replaced other minerals (prehnite, 
analcime, laumontite). Under the microsco[)e it is recognisable from quartz by its 

* * Die PalSolitiache Eruptivgesteine des Fichtelgebirges,' 1874, p. 29. See Rosenbusch, 
Mik. Physiog, iL p. 836. De la Vallee Poussin and Renanl, Mem, Couronnks Acad. Roy. 
de Bdffique^ 1876, xl. Plate vi. pp. 34 and 35. Fouque and Michel-Levy, • Mineralogie 
Micrograph,* p. 426. See postea, p. 618. 



72 GEOGNOSY book ii 

eharactoiistic i-ectaiigular fonns, cleavage, t\«'iiming, angle of extinction, turbidity, and 
frequent alteration.* Oithoclase weathers on the whole with comparative rapidity, 
though durable vaneties are known. Tlie alkali and some of the silica are removed, 
and the minei-al jtasses into clay or kaolin (p. 77). 

S a n i d i n c, the clear glassy fissured variety of orthoclase so conspicuous in the more 
silicated Tertiary and modem lavas, occurs in some trachytes in large flat tables (hence 
the name "sanidine") ; more commonly in fine clear or grey crystals or crystalline 
gi-anules ; an eminently volcanic mineral. 

Plagioclase (Triclinic) Felspars. — While the different felsi>ars which crystallize in 
the triclinic system may be more or less easily distinguished in large cr^'stals or 
crystalline aggregates, they ai-e difficult to se})arate in the minute forms in which they 
conmionly occur as rock constituents. Tliey have been gi*ou])ed by ];>etrographers under 
the general name Plagioclase (with oblique cleavage), proposed by Tschermak, who 
regards them as mixtures in vaiious projjortions of two fundamental comi>ounds — albite 
or so<la-fels[)ar, and anorthite or lime-felsi>ar. 

They occur mostly in well-develoj^ed ciystals, jwtrtly in irregular crj'stalline gn&ins, 
crystallites or microlites. On a fresh frat^ture, their crystals often a]q)ear as clear 
glassy striiKH, on which may usually be detected a fine ]>arallel lineation or ruling, 
indicating a characteristic polysynthetic twinning which never a])]>ears in orthoclase. 
A felspar striated in this manner can thus be at once pronounced to be a triclinic form, 
though the distinction is not invariably present. Under the microscojie, the fme 
] parallel lamellation or stri])ing, best seen with [solarized light, forms one of the most 
distinctive features of this group of felspars. The chief tiiclinic felsj^ars are, Microcline 
([lotash- felspar, K^Al-jSi^Oje), which occurs in gi*anites, jjaitieularly as the common 
fels{)ar of the graphic varieties ; also in some gneisses, &c. ; Albite (soda-fels^tar, NasO, 
11-82, Al-jOs 18'56, Si.jO 68'62), found in some granites, and in several volcanic rocks: 
01igo<rlase (soda-lime and lime-soda felsi)ars, Na^O 8-2, CaO 4-8, AljOs 23-0, SiOj 62-8) 
occurs in many granites and other eruptive rocks ; Andesine (NaoO 7*7, CaO 7*0, 
AloOs 26'6, Si0.j 60'0) observed in some syenites, &c. ; Labradoiite (XajO 4 '6, CaO 
12-4, AI2F3 30-2, SiOo 52'9), an essential constituent of many lavas, Ac, abundant in 
masses in the azoic rotrks of Canada, &c. ; Anorthite (lime-felsjwir, CaO 20*10, AljO, 
36'82, SiOo 43«08) found in many volcanic rocks, sometimes in gi-anites and metamori>hic 
rocks. 

Tlie triclinic felsi»ars have l>een ])roduced sometimes directly from igneous fusion, as 
can bo studied in many lavas, whei-e often one of the fii-st minei-als to apitear in the 
devitrification of the original molten glass has been the labradorite or other ]>lagioclas(i. 
In other cases, they have resulted from the ojHjration of the jn-ocesses tx) which the fonuation 
of the crystalline schists was due ; large beds as well as abundant diffused strings, 
veinings, and ciystals of triclinic felsjiar (labradorite) fonu a markerl feature among tlie 
ancient gneisses of Eastern Canada. The more highly silicated si>ecies (albite, oligoclase) 
(Kcur with orthoclase as essential (Constituents of many gi'anites and other plutonic rocks. 
The more basic forms (labiwlorite, anorthite) are generally absent where free silica is 
present ; but occur in the moi-e basic igneous rocks (basalts, &c.) 

Considerable differences are pi-esented by the triclinic fels^virs in regard to weathering. 
On an exposed face of rock they lose their glassy lusti^e and l»ecome white and opa<|ue. 
Tliis change, as in orthoclase, arises from loss of bases and silica, and from hydration. 
Traces of carbonates may often b<» oliserved in weathered crystals. Tlio oiiginal steam 
cavities of old volcanic rocks have generally been filled with infiltrated mineiBls, which 
in many cases have resulted from the weatheiing and decomiKisition of tlie triclinic 
felspars. Calcite, pi-elinite, and the fannly of zeolites have lK»en abundantly i^roduced in 
this way. The student will usually obsei-ve that where these minerals aliound in the 

* On niicroscoi)ic determination of felsjiars, see Fouqut* and Michel-Levy, op, cU. pp. 
209, 227, andi;(M/«/, pp. 94-90. 



PART n § ii EOCK'FORMIXG MINERALS 73 



cells and crevices of a rock, the rock itself is for the most i»art proi»ortionately deconi- 
l>osed, showing the relation that subsists between iutiltration-products and the decom- 
position of the surrounding mass. Abundance of calcite in veins and cavities of a fel- 
s]iathic rock affords good ground for 8usi>ecting the ju'esence in the latter of a lime 
felspar.^ (See under **Albitization," />oa^a, p. 618.) 

Saussurite, formerly described as a distinct mineral S]>ecies, is now found to be the 
result of the decomposition of felsiMirs, which have thus acquired a dull white asjiect and 
contain secondary crystallizations (zoisite) out of the decomiK^sed substance of the 
original felspar. Such saussuritic felsjiara occur in varieties of gabbro and diorite. Under 
the microscope they present a confused aggregate of crystalline needles and granules 
imbedded in an amoriihous matrix. (See postca, p. 618.) 

Lsneite (K]0 21-53, AlgO^ 23*50, SiOa 54-97) is a markedly volcanic mineral, occiu-- 
ring as an abundant constituent of many ancient and modem Italian lavas, and in some 
varieties of basalt. Under t^^e micro8Coi)e, sections of this mineral are eight-sided or 
nearly circular, and very commonly contain enclosures of magnetite, &c., confoiining in 
arrangement to the external fonn of the crystal or disjwsed radially. 

Hepheline (NajO 17-04, AI2O3 35-26, KjO 6-46, SiOa 41-24), essentially a volcanic 
mineral, l)eing an abundant constituent of ])honolite, of some Vesu\nan lavas, and of 
some forms of basalt, presents under the microscoi* various six-sided and even four-sided 
forms, according to the angles at which the prisms are eut.^ Under the name of Elmolite 
are comprised the greenish or reddish, dull, gi-easy-lustred, conijiact or massive varieties 
of nepheline, which occur in some syenites and other ancient crystalline rocks. 

The Mica Family embraces a nimil)er of minerals, distinguished esi»ecially by 
their very perfect basal cleavage, whereby they can lie split into remarkably thin elastic 
laminfe, and by a predominant si)leiident })early lustre. They consist essentially of 
silicates of alumina, magnesia, iron and alkalies, and may be conveniently divided into 
two groujis, the white uiicaSy which are silicates of alumina with alkalies, iron and mag- 
nesia, and the black micas, in which the magnesia and iron jilay a more con8i)icuous part. 

MuBCOTite (Potash-mica, white mica, Glimmer, Kfi 3-07-12-44, Na-jO 0-4-10, FeO 
0-1-16, FeaO, 0-46-8-80, MgO 0-37-308, AI2O3 28.05-3841, SiOa 43-47-51.73, HaO 0-98- 
6-22), abimdant as an original constituent of many crystalline rocks (granite, &c. ), and 
as one of the characteristic minerals of the crystalline schists ; also in many sandstones, 
where its small [parallel flakes, derived, like the suiTounding quartz grains, fiom older 
crystalline masses, im]iart a silvery or "micaceous" lustre and fissility to tlie stone. ^ 
The persistence of muscoWte under exjwsure to weather is shown by the silver}' plates of 
the mineral, which may lie detected on a crumbling surface ofgrjiniteor schist where 
most of the other minerals, save the quartz, have decayed ; also by the fre(piency of the 
micaceous lamination of sandstones. 

--» Biolite (Magnesia-mica, black mica, MgO 10-30 per cent) occui-s abundantly as an 
original constituent of many granites, gneisses, and schists ; also sometimes in basalt, 
trachyte, and as ejected fragments and crystals in tufl". Tts small scales, when cut trans- 
verse to the dominant cleavage, may usually l>e detected under the microscoi)e by their 
remarkably strong dichroism, their fine i>arallel lines of cleavage, and their frequently 
frayed appearance at the ends. Under the action of the weather it assumes a i>ale, dull, 
soft crust, owing to removal of its bases. The inineral rufiellav, which occui-s in hexa- 
gonal brown or red opaque inelastic tables in some basalts and other igneous rocks, is 
regarded as an altered form of biotite. 

* A valnable essay on the stages of the weathering of triclinic felspar as revealed by the 
microtoope was published by G. Rose in 1867. Zeitsch. Deutsch, Ocd, Oes. xix. \\ 276. 

* On the microscopic distinction between nepheline and apatite, see Fouque and Michel- 
Levy, * Mineral. Micrograph. ' p. 276. 

* On the microscopic determination of the micas, see Fouqu6 and Michel-Levy, op, cit. 
p. 333. 



74 GEOGNOSY book ii 

Phlogopit^ is another dark ferro-magnesiaii mica wliich contains a little fluorine. 
Lepidolitc (Lithia-mica), o(*eur8 in some granites aiid crystalline schists, especially in 
veins. Daniourite, merely a variety of muscovite, occurs among crystalline schists. 
ScriciU, a talc-like variety of museovite, occurs in soft inelastic scales in many schists, 
as a result of the alteration of orthoclase felsi)ar.^ Margarodite, a silvery talc-like 
hydrous mica, is widely diffused as a constituent of granite and other crystalline rocks. 
ParayoniU, a scaly micaceous mineral, forms the main mass of certain alpine schists. 

Hornblende (Monoclinic Amphibole, CaOa 10-12, MgO 11-24, FeaOj 0-10. K\fi^ 
5-18, Si02 40-50 also usually with some Na^O, K,0 and FeO). Divided into two grou|i6. 
1st. Non- aluminous, including the white and pale green or grey fibrous varieties 
(tremolite, actinolite, kc, ) 2nd. Aluminous, embracing the more abundant dark green, 
brown, or black varieties. Under the microscope, hornblende presents cleavage-angles 
of 124° 30', the definite cleavage-planes intersecting each other in a well-marked lattice 
work, sometimes ^ith a finely fibrous character superadded. It also shows a marked 
pleochroism with polarized light, which, as Tschermak first pointed out, usuaUy 
distinguishes it from augite.' Hornblende has abundantly resulted from the alteration 
(paramori)hisni) of augite (see l)elow, Uralite). In many rocks the ferro-magneaian 
silicate which is now hornblende was originally augite ; the epidiorites, for instance, 
were probably once dolerites or allied pyroxenic rocks. The pale non-aluminous horn- 
blendes are found among gneisses, crystalline limestones, and other metamorphic rocks. 
The dark varieties, though also found in similar situations, sometimes even forming entire 
masses of rock (amphibolite, honiblende-rock, hornblende-schist), are the common forms 
in granitic and volcanic rocks (syenite, diorite, honiblende-andesite, &c.) The former 
group naturally gives rise by weathering to various hydrous magnesian silicates, notably 
to serjientine and talc. In the weathering of the aluminous varieties, silica, lime, 
magnesia, and a jwrtion of the alkalies are removed, with convei'sion of jiart of the 
earths and the iron into carbonates. The further oxidation of the ferrous carbonate is 
shown by the yellow and brown crust so commonly to be seen on the surface or 
j)enetiuting cracks in the hornblende. The change proceeds until a mere internal 
kernel of unalterwl mineral remains, or until tlie whole has been converted into a 
ferruginous clay. 

Anthophyllite (Rhombic Anijihibole (MgFe)Si03) is a mineral which occurs in 
bladed, sometimes rather fibrous fonns, among the more Imsic ]>ai-tjs of old gneisses ; also 
in zone^ of alteration round some of the ferro-magnesian minerals of certain gabbros. 

Soda-amphiboles resemble orrlinary hornblende, but, as their name denotes, they 
contain a more marked proportion of soda. They include a lilue variety* called 
GlaucopJuinCj which is found abundantly in certain schists ; RieheckUe^ which is also 
blue and occurs in some granites and micro-granites ; Ar/irdsonitCy a dark greenish or 
brown variety. 

Uralite is the name given to a mineral which was onginally pyroxene, but has now by 
a process of j»arani()r})hism acquired the internal cleavage and structure of hornblende 
(amphibole). Under the niicroscoi>e a still unchanged kernel of jiyroxene may in some 
sjiecimens l>e observed in the centre of a crystal sniTounded by strongly pleochroio horn- 
blende, with its characteristic cleavage and actinolitic needles {postcOy j). 617). SmaragdUe 
is a beautiful grass-green variety also resulting from the alteration of a pyroxene. 

Augite (Monoclinic Pyroxene, CaO 12-27-5, MgO 3-22-5, FeO 1-34, FcjO, 0-10, 
AI2O3 0-11 ; Si0.j 40-57-4). Divided like hornblende into two groups. 1st. Non- 
aluminous, >vith a j»revalent green colour (malacolite, coccolite, dio|>side, salUite, Ac) 
2n<l. Aluminous, including generally the dark gi-een oi* black varieties (common augite, 
fassaite). It would a]){iear that the substance of hornblende and augite is dimorphous, 

^ On the occurrence of this mineral in schists, see Lessen, Zeitsch. Deuiach. Oecl. Ot». 
1867, pp. 546, 661. 

- U7fw. Acad. May 1869. See also Fouque and Michel-Levy, op, cit, pp. 349, 865. 



PART II § u ROCK'FORMING MINERALS 75 

for the experiments of Berthier, MitHcherlich and G. Rose showed that hornblende, 
when melted and allowed to cool, assumed the crystalline form of augite ; whence it has 
been inferred that hornblende is the result of slow, and augite of comparatively rapid 
cooling.^ Under the microscope, augite in thin slices is only very feebly pleochroic, and 
presents cleavage lines intersecting at an angle of 87° 5'. It is often remarkable for the 
amount of extraneous materials enclosed within its crystals. Like some felspars, augite 
may be found in basalt with merely an outer casing of its own substance, the core being 
compoeed of magnetite, of the ground-mass of the surrounding rock, or of some other 
mineral (Fig. 7). The distribution of augite resembles that of hornblende ; the pale, 
non- aluminous varieties are more specially found among gneisses, marbles, and other 
crystalline, foliated, or metamorphic rocks ; the dark-green or black varieties enter as 
essential constituents into many igneous rocks of all ages, from Paleeozoic up to recent 
times (diabase, basalt, andesite, kc ) Its weathering also agrees >vith that of hornblende. 
The aluminous varieties, containing usually some lime, give rise to calcareous and 
ferruginous carbonates, from which the fine interstices and caWties of the surrounding 
rock are eventually filled with threads and kernels of calcite and strings of hydrous ferric 
oxide. In basalt and dolerite, for example, the weathered surface often acquires a rich 
yellow colour from the oxidation and hydration of the ferrous oxide. 

OmpkaeiUy a granular variety of pyroxene, grass green in colour, and commonly 
associated with red garnet in the rock known as eclogite. 

Diallagf, a variety of augite, characterised by its somewhat metallic lustre and 
foliated aspect, is especially a constituent of gabbro. 

BbomUc-Pjrrozflnss. — Tliere are three rhombic forms of )>}Toxene, which occur as 
important constituents of some rocks, Enstatite, Bronzite and Hypersthene. EnstaiiU 
occurs in Iherzolite, serpentine, and other olivine rocks ; also in meteorites. Brmvdte is 
found under similar conditions to enstatite, from which it is with difficulty seiiarable. 
It occurs in some basalts and in seqientiues ; also in meteorites. Bronzite and enstatite 
weather into dull green serpentinous pixxlucts. BastUe or Schiller-siiar is a frequent 
product of the alteration of Bronzite or Enstatite, an<i may l)e observed with its 
characteristic pearly lustre in 8eri»entine. Hypersthene occurs in hy]ier8thenite and 
hypersthene -andesite ; also associated vnth other magnesian niineralH among the 
crystalline schists. 

OliTiiie (Peridot, MgO 32.4-50.5, FeO 6-29.7, SiOj 31.6-42-8) fonns an essential 
ingredient of basalt, likevise the main {lart of various so-called olivine-rocks or perido- 
tites (as Iherzolite) and pikrite), and occurs in many gabbros ; under the microsco])e with 
polarized light, gives, when fresh, bright colours, specially re<l and green, but it is not 
perceptibly pleochroic. Its orthorhombic outlines can sometimes be readily observed, 
but it often occurs in irregularly shaped granules or in broken (;rystals, and is liable to 
be traversed by fine fissures, which are jwrticularly developed transverse to the vertical 
axis. It is remarkably prone to alteration. The change begins on the outer sui'face and 
extends inwards and siiecially along the fissures, until the whole is converted either 
into a green granular or fibrous substance, which is probably in most cases seri)entine 
(Fig. 26), or into a reddish-yellow amor|)hous mass (limonite). 

HanyiM (SiOa 84-06, Al 27-64, NajO 11.79, KjO 496, CaO 10-60, SO4 11-25) occurs 
abundantly in Italian lavas, in basalt of the Eifel, and elsewhere. 

HosMn (SiO, 33.79, Al 28-75, Na^O 26-20, SO4 11-26), under the niicroscoi>e, is one 
of the most readily recognised minerals, showing a hexagonal or quadrangular figure, 
with a characteristic broad dark bonier conesiionding to the external contour of the 
crystal, and where weathering has not jnoceeded too far, enclosing a clear colourless 
centre. It occurs in minute forms in most j)honolites, also in large cr^'stals in some 



^ The same results have been obtained recently by Fouque anfl Michel- Levy, 'Synthase 
des Mineranx et des Roches,' 1882, p. 78. 



76 GEOGNOSY book ii 

sHiiidiiie volcanic rocks. Both hauvuc and nosoan are volcanic minerals a&M)ciated 
with the lavas of more recent geological jieriods. 

Epidote (Pistacite, CaO l»-30, MgO 0-49, Yefis 7-5-17-24, AI3O, 14.47-28.9, 
Si0.j d3*81-57*65) occurs in many crystalline rocks, as a i-esult of the alteration of 
other silicates such as fels[Mirsand hornblende {see postea^ p. 618) ; largely distributed in 
certain schists and quaitzites, sometimes associated with beds of magnetite and haematite. 

Zoisite is allied to e])idote but contains no iron. It occurs in altered basic igneous 
ix)cks and also (sometimes in large aggregations) in metamorjihic grou{)8. 

VesuTianite (Idoci-ase, CaO 27-7-37-5, MgO 0-10-6, FeO 0-16, AI0O3 10-5-26.1, 
SiOa 35-397, H2O 0-2*73) occurs in ejected blocks of altei*ed limestone at Somma, 
also among crystalline limestones and schists. 

Andalosite (AI2O3 50-96-62-2, Fe-jOs 0-5-7, SiO-j 35.3-40-17).— Found in crystal- 
line schists. The variety Chiastolite, abundant in some dark clay-slates, is dis- 
tinguished by the regular manner in which the dark sultstance of the surrounding matrix 
has been endased, giving a cross-like transverse section. These crystals have been 
develojjed in the rock after its formation, and are regarded as proofs of contact-meta- 
morphism. (Book IV. Part VIII.) Silliinanile or Fibrolitc is the name given to a 
til)rous variety which is not infrequent among schistose rocks. 

Dichroite (Coitlierite, lolite, MgO 8-2-20-45, FeO 0-11-58, AI2O3 28.72-33-11, 
SiOo 48-1-50-4, HuO 0-2-66) occura in gneiss, sometimes in large amount (cordierite- 
gneiss) ; occasionally as an accessory ingredient in some gi-anites ; also in talc-schist. 
Uuilergoes numerous alterations, having been found changed into ]»inite, chloro[)liyllite, 
mica, &c. 

Scapolites, a series of minerals consisting of silicates of alumina, lime and soda, with a 
little chlorine. They are found among the cavities of lavas, but more frec[uently among 
metamorphic rocks, where they appear in association with altered fels]»ars. Dipyre, Cou- 
arranUc and Mcionite are varieties of the series. 

Kyanite ( Alj SiOg) occurs in bladed aggregates of a beautiful delicate blue colour among 
scOiistose rocks ; also in gi*anular foims. 

Garnet (CaO 0-5-78, MgO 0-10-2, Fe^Og 0-6-7, FeO 24-82-39-68, MnO 0-6-43, 
AI0O3 15-2-21-49, SiOa 35-75-52-11). — The common red and Inowni varieties occur as 
essential constituents of edogite, garnet- rock ; and often as abundant accessories in mica- 
schist, gneiss, granite, &c. Under the microscope, garnet as a constituent of rocks, 
l>resents three-sided, four-sided, six-sided, eight-sided (or even rounded) figures according 
to the angle at which the indi\ndual crystals are cut ; it is usually clear, but full of llavv's 
or of cavities ; [lassive in jM>larized light. 

Tourmaline (Schori, CaO 0-2*2, AlgO 0-14*89, Na-^O 0-4*95, KoO 0-3*59, FeO 0-12, 
Fe-jOs 0-1308, AljOg 30*44-44*4, SiOj 35*2-41*16, B 3*63-11 78, F 1*49-2*58), with 
quartz, fonns tounnaline-rock ; associated with some granites ; occurs also diffused 
tlirough many gneisses schists, crystalline limestones, and dolomites, likewise iu sands 
(see Zireon). Pleo(;hroism strongly marked. 

Zircon (ZrOa 63*5-67*16, FcjOj 0-2, SiOo 32-35*26) occurs as a chief ingredient 
in the zircon-syenite of Southern Norway ; frequent in granites, diorites, gneisses, crystal- 
line limestones and schists ; in eclogite ; as clear red grains in some basalts, and also iii 
ejecte<l volcanic blocks ; of common ocininence in san<ls, clays, sandstones, shales and 
other sedimentaiy nxiks derive<l from ciystalline ma.sses such as graiut<», etc. 

Titanito (Sphene, CaO 21*76-33, TiOa 33-43*5, SiO-j 30-35), dlsjiersed in small 
chaiTurteristically lozeng<^shai)cd ciystals in many syenites, also in gi*anite, gneiss, and 
in some volcanic rocks (l>asalt, trachyte, j»honolite). 

Zeolites. — Under this name is included a characteristic family of minerals, which have 
resulted fix>m the alteration, and jMii-ticularly from the hydration, of other minerals, 
esi)ecially of felsjiars. Secondary products, rather than original constituents of rocks, 
they often occur in caWties lK)th as prominent amygdales and veins, and in minute 
interstices only ^lereeptible by the microscojw. In these minute forms they ver}' commonly 



PARTUgii ROOK-FORMING MINERALS 77 

present a finely fibrous divergent structure. As already remarked, a relation may often 
be traced between the containing rock and its enclosed zeolites. Thus among the basalts 
of the Inner Hebrides, the dirty green decomposed amygdaloidal sheets are the chief 
repositories of zeolites, while the firm, compact, columnar beds are conqiaratively free 
from these alteration products.* Among the more common zeolites are Analcinic, 
NcUrciiU, PrehnUe and StUbite, 

Kaolin (AI3O, 38-6-40-7, CaO 0-3'5, KjO 0-19, SiOj 45 -6-46 53, H2O 9-14*54) 
results from the alteration of {K>tash- and soda-felspars exposed to atmospheric influences. 
Under the microscope the fine white powdery substance is found to include abundant 
minute six-sided colourless plates and scales which have been formed by re-crystallization 
of the decomposed substance of the felspar. The purest white kaolin is called china-clay^ 
from its extensive use in the manufacture of porcelain. Ordinary clay is impui'e from 
admixture of iron, lime, and other ingredients, among which the debris of the unde- 
composed constituents of the original rock may form a marked pro] tort ion. 

Talc (MgO 23-19-35-4, FeO 0-4-5, Al,Os 0-5*67, SiOj 56*62-64*53, HjO 0-6*65) 
occurs as an essential constituent of talc-schist, and as an alteration product replac- 
ing mica, honiblende, augite, olivine, diallage, and other minei*als in crystalline 
rocks. 

CShlorite (MgO 24*9-36, FeO 0-5*9, FeaOs 0-11*36, AI2O3 10*5-19*9, SiOo 30-33*5, 
H5O 11 '5-16), including several varieties or si)ecie8, occurs in small green hexagonal 
tables or scaly vermicular or earthy aggregates ; is an essential ingredient of chlorite- 
achist, and occurs abundantly as an alteration product (of honiblende, &c.) in fine 
filaments, incrustations, and layers in many crystalline rocks. (See under '* Chloritiza- 
tion,*' ;N»^ea, p. 618.) Among the minerals grouj)ed under the general head of chlorites 
are Chlcrophonte^ Cliiwchlorey Deles^Uc, Pennine ^ RipidolitCf and othera. 

(Htrelite (Chloritoid, H2O (FeMg) AlaSiOy) occurs in small lustrous iron-black or 
greenish-black lozenge-shai)ed or six-side plates in certain schists. It resembles chlorite 
Imt is at once distinguishable from that mineral by its much greater hardness. 

Sexpentine (MgO 28-43, FeO 1-10*8, AI2O3 0-5*5, SiO^ 37*5-44*5, H2O 9*5-14*6) is a 
product of the alteration of pre-existing minerals, and especially of olivine. It occurs 
in nests, grains, threads, and veins in rocks which once contained olivine * (p. 75), also 
massive as a rock, in which it has replaced olivine, enstatite or some other magnesian 
biailicate (pp. 173, 618). Under the microscope it presents, in very thin slices, a i>ale leek- 
green or bluish-green base, showing aggregate [polarization. Through this base runs a 
network of dark opaf^ue threads and veinings. Sometimes among these veinings, or 
throogh the network of green serpentinous matter in the base, the forms of original 
olivine crystals may be traced (Figs. 26, 27). 

Olanocmite (CaO 0-4*9, MgO 0-5*9, K^O 0-12*9, NayO 0-2*5, FeO 3-25*5, 
FeiQt 0-28*1, AljO, 1*5-13*3, Si02 46*5-60*09", H2O 0-14*7). Found in many strati- 
fied formations, particularly among sandstones and limestones, where it envelopes 
grains of sand, or fills and coats foraminifera and other organisms, giving a general 
green tint to the rock. It is at present being formed on the sea-floor off the coasts of 
Georgia and South Carolina, where Pourtaleir found it filling the chambers of recent 
polythalamia. 

6. Carbonates. This family of minerals furnishes only four which enter largely 
into the formation of rocks, viz., Carbonate of Calcium in its two forms, Calcite and 
Aragonite, Carbonate of Magnesium (and Calcium) in Dolomite, and Carbonate of Iron in 
Siderite. 

Calcite (CaCOs) occurs as (1) an original constituent of many aqueous rocks (lime- 
stone, calcareous shale, Ac. ), either as a result of chemical deposition from water (calc- 



' See Sullivan in Jukes' * Manual of Geology,' p. 85. 
* See Tschermak, Wien, Akad, Ivi. 1867. 



78 GEOGNOSY book n 

sinter, stalactites, &c. ), or as a secretion by ])lants or animals ^ ; or (2) as a secondary 
product resulting from n^eathering, when it is found filling or lining cavities, or diffused 
through the capillary interstices of minerals and rocks. It probably never occurs as an 
original ingredient in the massive crystalline rocks, such as granite, felsite, and lavas. 
Under the niicrosco|je, calcite is readily distinguishable by its intersecting cleavage lines, 
by a fi-equent twin lamellation (sometimes giving interference colours), strong double 
refraction, weak or inappreciable ])leochroism, and characteristic iridescent polarization 
tints of grey, rose and blue. 

From the readiness with which water absorbs carbon -dioxide, from the increased 
solvent power which it thereby acquires, and from the abundance of calcium in various 
forms among minerals and rocks, it is natural that calcite should occur abundantly as a 
}iseudomorph replacing other minerals. Thus, it has been ob8er>'ed taking the place of a 
number of silicates, as orthoclase, oligoclase, garnet, augite and several zeolites ; of the 
sul[>hate8, anhydrite, gypsum, barytes, and celestine ; of the carbonates, aragonite, 
dolomite, cerussite ; of the fluoride, fluor-spar ; and of the sulphide, galena. Moreover, 
in many massive crystalline rocks (diorite, dolerite, &c.), which have been long 
exposed to atmospheric influence, this mineral may be recogiused by the briak 
effervescence productni by a drop of acid, and in niicroscojnc sections it appears filling the 
crevices, or sending minute veins among the decayed mineral constituents. Calcite is 
likcM-isc the great i)etrifying medium : the vast majority of the animal remains found 
in the rocky cnist of the globe have been replaced by calcite, sometimes with a com- 
plete preservation of internal organic structure, sometimes with a total substitution 
of cr^'stallinc material for that structure, the mere outer form of the organism alone 
surviving. - 

Aragonite (CaCO^), harder, heavier, and much less abundant than calcite, which is 
the more stable form of calcium-carbonate ; occurs with l)eds of g^'jxsum, also in mineral 
veins, in strings ninning tlirough basalt and other igneous rocks, and in the alielU of 
many mollusca. It is thus always a de|)osit from water, sometimes from warm mineral 
springs, sometimes as the result of the internal alteration of rocks, and sometimes 
through the action of living organisms. Being more easily soluble than calcite, it has 
no doubt in many cases disa])peared from limestones origiually fonned mainly of 
aragonite shells, and has ])een replaced by the moi-e durable calcite, with a consequent 
destniction of the traces of organic origin. Hence what are now thoroughly crystalline 
limestones may have l>een formed by a slow alteration of such shelly deposits (p. 484). 

Dolomite (Bitter-spar (Ca ; Mg)C03, ]i. 151) occurs (1) as an original deposit in 
massive IkmIs (magnesian limestone), belonging to many different geological formationa ; 
(2) as a product of alteration, esi)ecially of oixiinary limestone or of aragonite (Dole- 
mitizatiou j). 321). 

Siderite (Brown Ironstone, Si)athic Iron, Chalybite, Ferrous Carbonate, FeCO,) 
occurs crystallized in association with metallic ores, also in beds and veins of many 
crystalline rocks, jwirticularly with limestones ; the comi)act argillaceous varieties (clay- 
ironstone) are found in abundant nodules and beds in the shales of Carboniferous and 
other formations where they have been dejMJsited from solution in water in presence of 
decaying organic matter (see pp. 147, 153). 

7. SuLPHATJiis. Among the sulphates of the mineral kingdom, only two deserve 
noticiJ here as inii»ortant comjiounds in the constitution of rocks — \'iz., calcium-sulphate 



^ Mr. Sorby has investigated the condition in which the calcareous matter of the 
harder parts of invertebrates exists. He finds that in foraminifera, echinoderms, 
brachiopods, Crustacea, and some lamellibranchs and gasteropods, it occurs as calcite ; 
that in nautilus, sepia, most gasteropods, many lamellibranchs, &c., it is aragonite ; and that 
in not a few cases the two forms occur tc^ether, or that the carbonate of lime is hardened by 
an admixture of pho8i>hate. Quart. Joum. Oeol. Soc. 1879. Address, p. 61. 

^ See index sub voc. Calcite. 



PAOT II § u ROCK-FORMING MINERALS 79 

or sulphate of lime in its two forms, Anhydrite and Gyjisum ; and bariuni-sul])hate or 
sulphate of baryta in Barytes. 

Anhydzite (CaSOi) occurs more esi>ecially in association witli beds of gy|)simi and 
rock-salt (see p. 152). 

Oypnim (Selenite, CaS04 + 2H20). Abundant as an original aqueous de|X)sit in 
many sedimentary formations (see p. 152). 

BazytM (Heavy Spar, BaSOJ. Frequent in veins and es|iecially associated with 
metallic ores as one of their characteristic vein-stones. 

8. Phosphates. The phosphates which occur most conspicuously as constituents or 
ac ceoD ory ingredients of rocks are the tricalcic phosphate or Ajiatite, and tri ferrous 
phosphate or Yivianite. 

Apatite (3Ca3 (POJ + CaF,) occurs in many igneous rocks (granites, Ijasalts, &c.), in 
minute hexagonal non-pleochroic needles, giving faint polarization tints ; also in large 
crystals and massive beds associated with metamorphic rocks. 

YiTimaite (Blue iron -earth, FesP^Og, SH^O) occurs crystallized in metalliferous 
veins ; the earthy variety is not infrequent in 2)eat-mosses where animal matter has 
decayed, and is sometimes to be observed coating fossil fishes as a fine layer like the 
bloom of a plum. 

9. Fluorides. The element fluorine, though widely diffused in nature, occurs as an 
important constituent of comparatively few minerals. Its most abundant com|K)und is 
with Calcium as the common mineral Fluorite. It occui*s also with sodium and 
almninium in the mineral Cryolite. 

Fluorite (Fluor-sjiar, CaFj) occurs generally in veuis, esiH?cialIy in association with 
metallic ores. 

10. Chlorides. Tliere is only one chloride of importance as a constituent of rocks 
— sodium-chloride or common salt (NaCl), which, occurring chiefly in beds, is described 
among the rocks at p. 148. Camallite (KClMgCl26H20), a liydrated chloride of 
potassium and magnesium, occurs in beds associated with rock-salt, gy[)sum, &c., in some 
salt districts (p. 149). 

11. Sulphides. Sulphur is found united with metals in tlie foi-m of sulpliides, 
many of which form common minerals. The sulphides of lead, silver, copper, zinc, 
antimony, Ac., are of great commercial imiwrtance. Iron-disulphide, however, is the 
only one which merits consideration here as a rock-fonuing substance. It is fonned at 
the present day by some thermal springs, and has been develoi»ed in many rocks as a 
result of the action of infiltrating water in presence of deconijwsing organic matter and 
iron salts. It occurs in two forms, Pyrite and Marcasite. 

P3frtte (Eisenkies, Schwefelkies, FeSo) occurs disseminated through almost all kinds 
of rocks, often in great abundance, as among dial>ases and clay-slates ; also frequent in 
Teins or in beds. In microscopic sections of rocks, jiyrite apjiears in small cubical, 
perfectly opaque crystals, which with reflected light show the characteristic brassy 
histre of the mineral, and cannot thus be mistaken for the isometric magnetite, of which 
the square sections exhibit a characteristic blue-black colom*. Pyrite when free from 
marcesite yields but slowly to weathering. Hence its cubical crystals may be seen 
projecting still fresh from slates which have been^ exi)osed to the atmosphere for several 
genermtions.^ 

Haicaiite (Hejiatic pyrites) occurs abundantly among sedimentar}' foimations, 
sometimes abundantly diffused in minute pai-ticles which inij)art a blue-grey tint, and 
speedily weather yellow on exposure and oxidation ; sometimes segregated in layers, or 
replacing the substance of fossil plants or animals ; also in veins through crystalline 
rocks. This form of the sulphide is especially characteristic of stratified fossiliferous 
rocks, and more particularly of those of Secondary and Tertiary date. It is extremely 



* For an elaborate paper on the decomposition of Pyrites, see A. A. Julien, Annals 
New York Acad, Set. vols. iii. and iv. 



^ 'V EO* ixos y BOOK II 



iU'-!«: t«^ «i»:».-'*ini*r-iii«iii. H»-n'.-»r exj-j^uri? f«.»r f vru a -hort linie to the air causes it to 
•«.•■#, rue bro'Aii : fr*-»- -liliiliiiri*; a^.i'l i* |in>liiiTeii. whii-h attai.-k-> the Mirroanding miDermls, 
-'irii»:tirii«7i at oii'.-^ fonuiu;; '-ulphate^. at oth*T tiiu*^ «i»r<^>m|<iHinjr aluminous ailicatm 
ari'i 'li^Vihiij^ t)i*rrii in •.-'tii-iileraM<r tfiiautity. Ih-. Sullivan mentions that the water 
iririii.AlIy puniii^l tWmi one mine in Inrlan<l >-arriH<l u|» !•■ the Mirface more than a 
;.iirj<lr*^i ton^ of t}i:v^Av*r*\ «ili<-ate of alumina.^ In'jn <lLMiI|ihiil«: i> thus an inifiortant 
ik'^ful in etf«-<-tin;; the int^-nial tltf^rfmiifr-xihtu of nArkiiw It al»o plays a large |«rt as a 
ji^trifyin;; ni«-<iiiini. rfplat-in^ t)ir onranio matter itf plant j^ an<l animals, and leaving 
«a-t-> of th»*ir fonn-*. •ift»-n with hri^ht niftallit.- lustre. Sm-h c-a>ts when exixiaied to the 

air f\*r*:'flU\ftr^. 

Pjrrrhotiiia Maaw-tu- ]*yrites. Fe^S^ i> mui.-h lr>« ahun<lant than either of the forms 
of onlinar}' irin-pyritt;-, fn>ni whiirli it i.> di>tin^ii«>he<l hy its inferior hardness and its 
ma^^netif «;Iianu:ter. 

It will lie observed that great differences exist in the relative im- 
IKirtiince of the minerals aliove enumerated as constituents of rocks. 
iVofertsor Kosenbusch jxiints out that they lUiiy be naturally arranged 
in four groujis — 1st, ores and accessory ingredients (magnetite, haematite, 
ilmenite, a{iatite, zircon, spinel, titanito), 2nd, magnesian and ferru- 
ginous silicates (biotite, amphibole, pyroxene, olivine), 3rd, felspathic 
constituents (felspar [)roj)er, ne]>heline. leucite, melilite, so<lalite, hauyne), 
4th, free silica.- 

§ Lil. Determination of Rocks. 

Kocks considered as mineral subsUmces are distinguished from each 
other by certain external characters, such as the size, form, and arrange- 
ment of their component jmrticles. These characters, readily perceptible 
to the naked eye, and in the great majority of cases observable in hand 
sjiccimcns, are termed mf(jascopk or inncmsct^pk (pp. 81-87), to distinguish 
them from the more minute features which, l)eing only visible or satis- 
factorily oV>8ervable when greatly nuignitied, are known as microscopic 
(pp. 89-96). The larger (geotectonic) aspects of rock-structure, which can 
only be proj^rly examined in the field and 1)elong to the general 
architecture of the earth's crust, are treated of in Book IV.^ 

In the discrimination of rocks, it is not enough to specify their 
comi>onent minerals, for the same minerals may constitute very distinct 
varieties of rock. For example, quartz and mica form the massive 
crystalline rock, greisen, the foliated crysUilline rock, mica-schist, and 
the sedimentary rock, micaceous sandstone. Chalk, encrinal limestone, 
stalagmite, statiuiry mar1)le arc all comiK)sed of calcite. It is needful 
to take note of the megascopic and microscopic structure and texture, 
the state of aggregation, colour, and other charactei*s of the several 
masses. 

1 JukeH' ' Manual of Geology,' j.. 65. ' Xcues JaJtrb. 1882 (iL) p. 6. 

* The student who would jmreue physical geology by original research in the field and 
abroad may consult Bouc, 'Guide da Geologue Voyageur,' 2 vols. 1835; £lie de Beaumont, 

* Le<;on8 de Geologie pratique/ vol. 1. 1845 ; Penuing and Jukes- Brown, 'Field Geology,' 
•2nd edit. 1880 ; A Geikie, ' Outlines of Field Geolog}-,' 4th edit 1891. F. v. Richthofen. 

* Fuhrer fiir Forschungsreisendc/ 1886 ; Grenville Cole, 'Aids in Practical Greology,' 1891. 



•ART II § iii DETERMINATION OF ROCKS 81 

Four methods of procedure are available in the investigation and 
le termination of rocks : 1st, megascopic (macroscopic) examination, either 
>7 the rough and ready, but often sufficient, appliances for use in the 
ield, or by those for more careful work indoors ; 2nd, chemical analysis ; 
Srd, chemical synthesis ; 4th, microscopic investigation. 

i. Megascopic {Macroscopic) Examination, 

Tefte in the field. — The instmmeuts indisi^usable for the investigation of rocks in 
:he field are few in number, and simple in character and application. The observer 
«rill be sufficiently accoutred if he carries with him a hammer of such form and weight 
iS will enable him to break off clean, sharp, unweathered chit)S from the edges of rock- 
naases, a small lens, a [)ocket-knife of hard steel for determining the hardness of rocks 
ind minerals, a magnet or a magnetized knife-blade, and a small i>ocket-phial of dilute 
lydrochloric acid, or better still some citric acid in powder. 

Should the object be to form a collection of rocks, a hammer of at least three or four 
[Monds in weight should be carried : also one or two chisels and a small trimming 
lammer, weighing about i lb., for reducing the specimens to shape. A convenient size 
>f specimen is 4x3x1 inches. They should be as nearly as possible uniform in size, 
M> as to be capable of orderly aiTangement in the drawers or shelves of a case or cabinet. 
/Attention should be jiaid not only to obtain a thoroughly fresh fracture of a rock, but 
ilso a weathered surface, wherever there is anything characteristic in the weathering. 
Every specimen should have affixed to it a label, indicating as exactly as possible the 
iocadity from which it was taken. This information ought always to be written down in 
the field at the time of collecting, and should be affixed to or wTap]>ed up with the 
specimen, before it is consigned to the collecting bag. If, however, the student does 
not purpose to form a collection, but merely to obtain such chips as will enable him to 
judge of the characters of i*ocks, a hammer weighing from 1^ to 2 lbs., with a square 
fkce and tai)ering to a chisel-edge at the opposite end, will be most useful. The advantage 
if this form is that the hammer can be used not only for breaking hard stones, but also 
Tor splitting open shales and other fissile rocks, so that it unites the uses of hammer and 
shiseL 

It is, of course, desirable that the learner should first acquire some knowledge of the 
nomenclature of rocks, by carefully studying a collection of correctly named and 
iudiciously selected rock-specimens. Such collections may now be purchased at small 
M)6t from mineral dealers, or may be studied in the museums of most towns. Having 
locustomed his eye to the ordinary external characters of rocks, and become familiar 
idth their names, the student may j>roceed to determine them for himself in the field. 

Finding himself face to face with a rock-mass, and after noting its geotectonic 
sharacters (Book IV.), the observer will proceed to examine the exposed or weathered 
mxiiyce. The earliest lesson he has to learn, and that of which perhaps he will in after 
ife meet with the most varied illustrations is the extent to which weathering conceals 
he true aspect of rocks. From what has been said in previous pages, the nature of 
lome of the alterations will be understood, and fmlher information regarding the 
chemical processes at work will be found in Book III. The practical study of rocks in 
he field soon discloses the fact, that while, in some cases, the weathered crust so 
Mnnpletely obscui-es the essential character of a rock that its true nature might not be 
inspected, in other instances, it is the weathered crust tliat best reveals the real 
(tructure of the mass. Spheroidal crusts of a decomposing yellow ferruginous earthy 
mbetance, for example, would hardly be identihed as a coniiwet dark basalt, j^et, on 
woetrating within these crusts, a central core of still undecomposed basalt may not 
infrequently be discovered. Agtiin, a block of limestone when broken open may 
iresent only a uniformly crjstalliue structure, yet if the weathered surface be examined 



82 GEOGNOSY book ii 

it may show many projecting fragments of shells, ]K)lyzoa, corals, criuoids, or other 
organisms. The i-eally fossiliferous nature of an ap])arently unfossiliferous rock may 
thus 1)e revealed by weathering. Many limestones also might, from their fresh fracture, 
be set down as tolerably pure carbonate of lime ; but from the thick crust of yellow 
ochre on their weathered faces are seen to be highly ferruginous. Among crystalline 
rocks, the weathered surface commonly throws light uix>n the mineral constitution of 
the mass, for some minerals decompose more rapidly than others, which are thus left 
isolated and more easily recognisable. In this manner, the existence of quartz in many 
fels|)athic rocks may be detected. Its minute blebs or crystals, which to the naked eye 
or lens are lost among the brilliant facettes of the felsjiars, stand out amid the dull clay 
into which these minerals are decomposed. 

The depth to which weathering extends should be noted. The student must not be 
too confident that he has reached its limit, even when he comes to the solid, more or less 
hard, splinteiy, and apiurently fresh stone. Granite sometimes decomposes into kaolin 
and sand to a depth of twenty or thirty feet or more. Limestones, on the other hand, 
have often a mere filni of crust, because their substance is almost entirely dissolved and 
removed by rain (Book III. Part II. Section ii. § 2). 

With some practice, the inspection of a weathered surface will frequently suffice to 
determine the tme nature and name of a rock. Should this ])reliminary examination, 
and a comx)arison of weathered and unweathered surfaces, fail to afford the information 
sought, we proceed to apply some of the simple and useful tests available for field-work. 
The lens "will usually enable us to decide whether the rock is compact and apparently 
structureless, or cr^'stalline, or fragmental. Having settled this point, we proceed to 
ascertain the hardness and colour of streak, by scratching a fresh surface of the stone.. 
A drop of acid placed ujion the scratched sui-face or on the powder of the streak may 
reveal the presence of some carbonate. By [iractice, considerable facility can be acquired 
in approximately estimating the si^ecific gravity of i-ocks merely by the hand. The 
following table may be of assistance, but it must be understood at the outset that a 
knowledge of rocks can never be gained from instructions givien in books, but must be 
actiuired by actual handling and study of the rocks themselves. 

i. A fresh fracture 8how8 the rock to be close-grained, daU, with no diitiBet 
Btractnre.^ 

o. H. 0*5 or less up to 1 ; soft, crumbling or easily scratched with the knife, if not 
with the finger - nail ; emits an earthy smell when breathed upon, does not 
effervesce with acid ; is dark grey, brown, or blue, perhaps red, yellow, or even 
white = probably some clay rock, such as niudstone, massive shale, or fire-clay 
(p. 132) ; or a decomjwsed felspar-rock, like a close-grained felsite or orthoclase 
t>orjihyry. If the rock is hard and fissile it may be shale or clay-slate {\u 
134). 
/9. H. 1*5-2. Occurs in Ijeds or veins (perhaps fibrous), white, yellow, or reddish. 
Sp. gr. 2 '2-2 '4. Docs not effervesce = probably gypsum (pp. 79, 152). 

7. Friable, cnimbling, soils the tiugere, white, or yellowish, brisk effervescences 

chalk, marl, or some pulverulent form of limestone (pp. 139, 149). 

8. H. 3-4. Sp. gr. 2 '5-2 '7 ; i)ale to dark green or reddish, or with blotched and 

clouded mixtures of these colours. Streak white ; feels soapy ; no effervescence, 
splintery to 8ul>conchoidal fracture, edges subtranslucent. See serpentine 
(p. 173). 
€. H. averaging 3. Sp. gr. 2 '6-2 '8. White, but more frequently bluish-grey, also 
yellow, brown and l)lack ; streak white ; gives brisk effervescence = some fomi 
of limestone (]>p. 139, 149). 

^ In this table, H. = hanlness ; Sp. gr. = specific gravity. The scale of hardness usually 
employed is 1, Talc ; 2, Rock-salt or gypsum ; 3, Calcite ; 4, Fluorite ; 5, Apatite ; 6» 
Orthoclase ; 7, Quartz ; 8, Topaz ; 9, Corundum ; 10, Diamond. 



PABT II § iii DETERMINATION OF ROCKS 83 



f. H. 3 '5-4 '5. Sp. gr. 2 '8-2 -95. Yellowish, white, or pale brown. Powder 
slowly soluble in acid with feeble effervescence, which becomes brisker when 
the acid is heated with the powder of the stone. See dolomite (pp. 78, 151). 
i|. H. 3-4. Sp. gr. 3-8*9. Dark brown to dull black, streak yellow to brown, 
feebly soluble in acid, which becomes yellow ; occurs in nodules or beds, 
usually with shale ; weathers with brown or blood-red crust = brown iron -ore. 
See clay-ironstone (pp. 147, 163) ; and limonite (pp. 70, 158) ; if the rock is 
reddish and gives a cherry-red streak, see haematite (pp. 70, 153). 
0, Sp. gr. 2*55. White, grey, yellowish, or bluish, rings under the hammer, splits 
into thin plates, does not effervesce, weathered crust white and distinct = 
perhaps some compact variety of phonolite (p. 166. See also felsite (p. 161), 
and porphyrite, p. 168). 
*• Sp. gr. 2 '9-3 "2. Black or dark green, weathered crust yellow or brown = 
probably some close-grained variety of basalt (p. 170), andesite (p. 167), 
aphanite (p. 166), or amphibolite (p. 182). 
K, H. 6-6*5, but less according to decomposition. Sp. gr. 2*55-2*7. Can with 
difficulty be scratched with the knife when fresh. White, bluish-grey, yellow, 
lilac, brown, red ; white streak ; sometimes with well defined white weathered 
crust, no effervescence = probably a felsitic rock (p. 161). 
X, H. 7. Sp. gr. 2*5-2*9. The knife leaves a metallic streak of steel upon the 
resisting surface. The rock is white, reddish, yellowish, to brown or black, 
very finely granular or of a homy texture, gives no reaction with acid = 
probably silica in the form of jasper, homstone, flint, chalcedony, halleflinta 
(pp. 69, 183), adinole (p. 183). 
iL A fireih fracture showt the rock to be glassy. 

Leaving out of account some glass-like but crystalline minerals, such as quartz and 
rock-salt, the number of vitreous rocks is comparatively small. The true nature of the 
mass in question will probably not be difficult to determine. It must be one of the 
ICaasive volcanic rocks (p. 15A et seq.) If it occurs in association with siliceous lavas 
(liparites, trachytes) it will probably be obsidian (p. 162), or pitchstone (p. 163) ; if it 
pionfo into one of the basalt-rocks, as so commonly happens along the edges of dykes 
and intrusive sheets, it is a glassy form of basalt (p. 171). Each of the three great 
series of eruptive rocks. Acid, Intermediate, and Basic, has its glassy varieties (see 
pp. 162, 163, 171). 

ilL A fresh fracture shows the rock to be crystaUine. 

If the component crystals are sufficiently large for determination in the field, they 
may suggest the name of the rock. Where, however, they are too minute for identifi- 
cation even with a good lens, the observer may require to submit the rock to more 
precise investigation at home, before its true character can be ascertained. For the 
purposes of field-work, however, the following points should be noted. 
a. The rock can be easily scratched with the knife. 

(a) Effervesces briskly with acid = limestone. 

(b) Powder of streak effervesces in hot acid. See dolomite (p. 151). 

{c) No effervescence with acid : may be granular crystalline gypsum (alabaster) 
or anhydrite (pp. 79, 152). 
fi. The rock is not easily scratched. It is almost certainly a silicate. Its character 
should be sought among the massive crystalline rocks (p. 154). If it be heavy, 
appear to be composed of only one mineral, and have a marked greenish 
tint, it may be some kind of amphibolite (p. 182) ; if it consist of some 
white mineral (felspar) and a green mineral which gives it a distinct green 
colour, while the weathered crust shows more or less distinct effervescence, it 
may be a fine-grained diorite (p. 165), or diabase (p. 170) ; if it be grey and 
granular, with striated felspars and dark crystals (augite and magnetite), with 
a yellowish or brownish weathered crust, it is probably a dolerite (p. 169) or 



84 GEOGNOSY book u 

andcsite (p. 167) ; if it be compact, finely-crystalliiie, scratched with difficulty, 
showing crystals of orthoclase, and witli a bleaclied argillaceous weathered 
crust, it is probably an orthoclase-porphyry (p. 164), or quartz-porphyry (p. 160). 
The occurrence of distinct blebs or crystals of quartz in the fresh fracture 
or weathered face will suggest a place for the rock in the quartziferous 
crystalline series (granites, quartz-porphyries, rhyolites), or among the g^eisMS 
and schists. 
iv. A fresh fracture shows the rock to have a foliated structure. 
The foliated rocks are for the most ]>art easily recognisable by the prominence of 
their component minerals (p. 175). Where the minerals are so intimately mingled as 
not to be separable by the use of the lens, the following hints may be of service : — 
a. The rock has an unctuous feel, and is easily scratched. It may be talc-schist 
(p. 183), chlorite - scliist (p. 183), sericitic mica -schist (p. 185), or foliated 
serpentine (p. 183). 
/3. The rock emits an earthy smell when breathed on, is harder than those included 
in a, is fine -'grained, dark-gi'ey in colour, splits with a slaty fracture and 
contains i)erhaps scattered crystals of iron -pyrites or some other mineral. It 
is some argillaceous-schist or clay-slate, the varieties of which are named from 
the predominant enclosed mineral, as chiastolite - slate, andalusite - schist, 
ottrelite-schist Ac. (p. 179) ; if it has a silky lustre it may be phyllite. 
y. The rock is comjwsed of a mass of ray-like or fibrous ci-ystals matted together. 
If the fibres are exceedingly fine, silky, and easily separable, it is probably 
asbestos ; if they are coarser, greenish to white, glassy, and hard, it is 
probably an actinolite-schist (p. 182). Many seri^en tines are seamed with veins 
of the fine silky fibrous variety termed chrysotile, which is easily scratched. 
8. The rock has a hardness of nearly 7, and splits with some difficulty along 
micaceous folia. It is probably a quartzose variety of mica-schist, quartz- 
schist, or gneiss (pp. 179, 184, 185). 
c. The rock shows on its weathered surface small ]>articles of quartz and folia of 
mica in a fine decomposing base. It is probably a fine-grained variety of 
mica-schist or gneiss. 
y. A fresh fracture shows the rock to have a frag^ental (clastic) structure. 
Where the component fragments are large enough to be seen by the naked eye or 
with a lens, there is usually little difficulty in dctennining the true nature and proper 
name of the rock. Two characters require to Ije specially considered — the component 
fragments and the cementing jMwte. 

1. The FragmetUs. — According to the sliajw, size, and comi)Osition of the fragments, 
different names are assigned to clastic rocks. 

a. Shape. — If the fi-agmeuts are chiefly rounded, the rock may be sought in the 
sand and gravel series (p. 127), while if they are large and angular, it may be classed 
as a breccia (p. 130). Some mineral substances, however, do not acquire rounded 
outlines, even after long-continued attrition. Mica, for example, splits up into thin 
laminae, which may bo broken into small flakes or spangles, but never become rounded 
granules. Other minerals, also, which have a ready cleavage, are apt to break up 
along their cleavage -i)lanes, and thus to retain angular contours. Calc-spar is a 
familiar example of this tendency. Organic i-emains comj^sed of this mineral (such as 
crinoids and echinoids) may often be noticed in a very fragmentary condition, having 
evidently been subjected to long-continued comminution. Yet angular outlines and 
fresh or little worn cleavage-surfaces may be found among them. Many limestones 
consist largely of sub-angular organic debris. Angular inorganic detritus is character- 
istic of volcanic breccias and tuffs (p. 135). 

/3. Size. — Where the fragments are hai-d, rounded, or sub-angular quartzose grains, 
the size of a pin's head or less, the rock is probably some form of sandstone (p. 131). 
Where they range up to the size of a i)ea, it may be a j>ebbly sandstone, fine con- 



PART II § iii DETERMINATION OF ROCKS 85 

glomerate or grit ; where they vary from the size of a pea to that of a walnut, it is an 
ordinary gravel or conglomerate ; where they range up to the size of a man's head or 
larger, it is a coarse shingle or conglomerate. A considerable admixture of sub-angular 
stones makes it a breociated conglomerate or breccia ; but where the materials are 
loosely aggregated, the deposit may be some kind of glacial drift, such as moraine-stuff 
or boulder-clay (p. 133). Large angular and irregular blocks are characteristic of coarse 
volcanic agglomerates (p. 137). 

y , Composition. — In the majority of cases, the fragments are of quartz, or at 
least of some siliceous and enduring mineral. Sandstones consist chiefly of rounded 
quartz-grains (p. 131). Where these are unmixed with other ingredients, the rock is 
sometimes distinguished as a quartzose sandstone. Such a rock when indurated 
becomes quartzite (p. 180). Among the quartz -grains, minute fragments of other 
minerals may be observed. When any one of these is prominent, it may give a name to 
the variety of sandstone, as felspathic, micaceous (p. 104). Volcanic tuffs and breccias 
are characterised by the occurrence of lapilli (veiy commonly cellular) of the lavas from 
the explosion of which they have been formed. Among interbedded volcanic rocks, the 
student will meet with some which he may be at a loss whether to class as volcanic, 
or as formed of ordinary sediment. They consist of an intermixture of volcanic detritus 
with sand or mud, and pass on the one side into tnie tuffs, on the other into sandstones, 
shales, limestones, etc. If the component fragments of a non-crystalline rock give a 
brisk effervescence with acid, they are calcareous, and the rock (most likely a limestone, 
or at least of calcareous composition) may be searclied for traces of fossils. 

2. The Paste, — It sometimes happens that the component fragments of a clastic 
rock cohere merely from pressure and without any discoverable matrix. This is 
occasionally the case mth sandstone. Most commonly, however, there is some cementing 
paste. If a drop of weak acid produces effervescence from between the component 
non-calcareous grains of a rock, the paste is calcareous. If the grains are coated with a 
red crust which, on being bruised between white i»ai>er, gives a cherry-red powder, the 
cementing material is the anhydrous peroxide of iron. If tlie jMiste is yellow or brown 
it is probably in great part the hydrous peroxide of iron. A dark brown or black 
matrix which can be dissipated by heating is bituminous. Where the comjwnent 
grains are so firmly cemented in an exceedingly hard matrix that they break across rather 
than separate from each other when the stone is fractured, tlie paste is j>robably siliceous. 

Detennination of Specific Gravity. — The student will find this character of con- 
siderable advantage in enabling him to discriminate between rocks. He may acquire 
some dexterity in estimating, even with the hand, the probable specific gravity 
of substances ; but he should l>egin by determining it with a balance. Jolly's spring 
balance is a simple and serviceable instrument for this purpose. It consists of an 
upright stem having a graduated strip of mirror let into it, in front of which hangs 
a long spiral wire, with rests at tlie l>ottom for weighing a substance in air and in 
water. For most purposes it is sufficiently accurate, and a determination can be made 
with it in the course of a few minutes.^ Another and more convenient instrument has 
been invented by W. N. Walker, consisting of a lever graduated into inches and tenths, 
and resting on a knife-edge stand, on one side of which is ]>Iaced a movable weight, 
while on the long graduated side the substance to l>e weiglied is suspended. This 
instrument has the advantage of not being so liable to get out of order as other 
contrivances.' 

^ Jolly's spring balance can be obtained through any optician or mineral dealer from 
Berberich, of Munich, for nine florins or 27s. In the United States it is manufactured 
bv Geo. Wade & Co., at the Hoboken Institute. 

* See Oeol, Mag, 1883, p. 109, for a description and drawing of this instrument, and 
the manner of using it. It may be obtained of Lowden, optician, Dundee, and How & Co., 
Farriogdon Street, London. Its price is 31 «. 6ti. 



86 GEOGNOSY book u 

Meohaaioal AnalyBis. — Much may be learnt regarding the composition of a rock 
by reducing it to powder. In the case of many sandstones and clays this reduction may 
easily be effected by drying the stone and crumbling it between the fingers. But when 
the material is too compact for such treatment some fragments of it placed within folds 
of paper upon a surface of steel may be reduced to |K)wder by a few smart blows of a 
hammer. The powder can be sifted through sieves of varying degrees of fmeness and 
the separate fragments may be picked out with a fine brush and examined with a lens. 
If they are dark in colour they may be placed on white paper, if light-coloured they 
are more readily observed upon a black paper. Portions of this ix>wder may be carefully 
washed and mounted with Canada balsam on glass, as in the way described below for 
microscopic slices. In this way the constituent minerals of many crystalline rooks may 
be isolated and studied with great facility. For purposes of comparison specimens of the 
rock-forming minerals should be procured and treated in a similar way. A series of 
typical preparations of the {K>wder or minute fragments of such minerals affords to the 
student an admirable basis from which to start in his study of the crystallographic and 
optical characters of the minerals which he will require to identify among the con- 
stituents of rocks. 

Another method of isolating the several components of certain rocks is by washing 
the triturated materials in water and alloiKing the sediment to subside. The finer 
and lighter ]>articles may be drawn off, while the coarser and heavier grains will sink 
according to their resi>ective specific gravities, and may then be separated and ooUeoted. 
This may be done by means of a ^ide tube with a stop-c^ck at the bottom, or by 
gently washing the powder with water on an inclined surface, when, as in the analogons 
treatment of veinstones and ores in mining, the particles arrange themselves acoordxng 
to their re8i>ective gravities, the lightest being swept away by the current. 

Magnetic particles may be extracted with a magnet, the end of which is preserved 
from contact witli the powder by being covered with fine tissue-|>aper. An electro- 
magnet will at once ^i-ithdraw the particles of minerals which contain far too little iron 
to be ordinarily recognised as magnetic ; in this way the particles of a ferruginous 
magiiesian mica may in a few seconds be gathered out of the powder of a granite.^ 

Where the difference between the specific gravity of the comjwnent minerals of a 
rock is slight, they may be separated by means of a solution of given density. Mr. £. 
Sonstadt proposed the use of a saturated solution of iodide of mercury in iodide of 
potassium, which has a maximum density of nearly 3 '2.*'' Rohrl)ach's solution, consisting 
of iodide of mercury and iodide of barium, has a density of as much as 3 '588.' More 
serviceable is the solution of borotungstate of cadmium, with a density of 3*28, proposed 
by D. Klein.^ The x)owder of a rock being introduced into one of these liquids, those 
particles whose sjiecific gravity exceeds that of the liquid will sink to the bottom, while 
those which are lighter will float. This ]>roce8s allows of the sc|)aration of the felspars 
from each other, and at once eliminates the heavy minerals such as hornblende, augite, 
and black mica. By the addition of water or other liquid, as the case may be, the 
si>ecific gravity may l>e reduced, and different solutions of given density may be emjdoyed 
for determining and isolating rock-constituents. This method of analysis is important 
in affording a ready means of se])arating the quartz and fels^Mir of a rock.'^ 

* Mim. AccuL. ties Sci. xxxiL No. 11 ; Fouque and Michel-Levy, ' Mindralogie Micro- 
graphique,* p. 115. 

•-^ Chem, News, xxix. (1874), p. 128. » Xeues Jahrb. 1888, p. 186. 

** Compt. rend, xciii. (1881), p. 318. More recently R. Brauns has introduced 
methylene iodide, which gives a density of 3*33 and is diluted with benzole. Neties Jahrb, 
1886, ii. p. 72. See also J. W. Retgers, op, cit. 1889, ii. p. 185. 

* Fouqu^ and Michel-L^vy, * Mineralogie Micrographicjue,' p. 117. Tlioulet, BidL Soc 
Afin, France^ ii. (1879), p. 17. A cheap form of instrument for isolating minerals by means 
of heavy solutions is described by Mr. J. W. Evans, Geol, Mag, 1891, p. 67. 



lRT n § iii DETERMINATION OF ROGKS 87 

Hydrofluoric add may be used in separating the mineral constituents of rocks. The 
ck to be studied is reduced to powder and introduced gently into a platinum capsule 
ntaining the concentrated acid. During the consequent effervescence, the mixture is 
ntiouBly stirred with a platinum spatula. Some minerals are converted into fluorides, 
hers into fluosilicates, while some, particularly the iron-magnesia species, remain 
idisaolved. The thick jelly of silica and alumina is removed with water, and the 
ystalline minerals lying at the bottom can then be dried and examined. By arresting 
le solution at different stages the different minerals may be isolated. This process is 
Imintbly adapted for collecting the pyroxene of pyroxenic rocks.* 

ii. Chemical Analysis, 

The determination of the chemical composition of rocks by detailed analysis in 
le wet way, demands an acquaintance with practical chemistry which comparatively 
w geologists possess, and is consequently for the most part left in the hands of chemists, 
lio are not geologists. But as some theoretical questions in geology involve a consider- 
»le knowledge of chemical processes, so a satisfactory analysis of rocks is best performed 
r one who understands the nature of the geological problems on which such an analysis 
■y be expected to throw light. As a rule, detailed chemical analysis lies out of the 
there of a geologist's work : yet the wider his knowledge of chemical laws and methods 
le better. He should at least be able to employ with accuracy the simpler processes of 
lemical research. 

TrtatmetU with Add. — The geologist's accoutrements for the field should include a 
aall bottle of powered citric acid, or one with a mineral acid, and provided with a 
ass stopper prolonged downwards into a point Dilute hydrochloric acid has been 
mmonly employed ; but H. C. Bolton proposed in 1877 the use of organic acids in place 
' the usual mineral acids. Citric acid is |)articularly serviceable for the purpose, and has 
le advantage over the mineral acids that it can be carried in powder, and a strong solution 
it in water can be made in such quantity and at such time as may be required. A 
;tle of the powder placed with the point of a knife on a surface of limestone and 
oistened ^^'ith a drop of water will give the proper reaction.* 

When a drop of acid gives effervescence upon a surface of rock, the reaction is caused by 
le liberation of bubbles of carbon dioxide, as this oxide is replaced by the more power- 
1 acid. Hence effervescence is an indication of the ])resence of carbonates, and when 
isk is specially characteristic of calcium-carbonate. Limestone and markedly cal- 
reoos rocks may thus at once be detected. By the same means, the decomposition of 
ch rocks as dolerite may be traced to a considerable distance inward from the surface, 
le original lime-bearing silicate of the rock having been decomposed by infiltrating 
in-water, and partially converted into carbonate of lime. This carbonate l)eing far 
ore sensitive to the acid-test than the other carbonates usually to be met with among 
cks, a drop of weak cold acid suffices to produce abundant effervescence even from a 
ystalline face. But the effervescence becomes much more marked if we apply the acid 
I the powder of the stone. For this purpose, a scratch may be made and then touched 
ith acid, when a more or less copious discharge of carbonic acid may be obtained, 
here otherwise it might ap|>ear so feebly as perhaps even to esca])e observation. Some 
Tbonates, dolomite for example, are hardly affected by acid until it is heated. This is 
)De by placing some fragments of the substance at the bottom of a test-tube, covering 
lem with acid and applying a flame. 

It is a convenient method of roughly estimating the purity of a limestone, to place a 
agment of the rock in acid. If there is much impurity (clay, sand, oxide of iron, &c. ), 
lis will remain behin'd as an insoluble residue, and may then be further tested chemi- 

* Fouqu4 and Michel-L^vy, op. cit. p. 116. 
* Ann. N€w York Acad. ScL i. (1879) p. 1. Chem. News, xxxvi.. xxxvii., xxxviii., xliii. 



88 GEOGNOSY book n 

pally, or examined with the microscoi^. In this way many limestones among the 
crystalline schists may l)e dissolved in acetic acid, leaving a residue of pyroxenes, amphi- 
boles, micas or other silicates. Of course the acid, e8i»ecially if strong mineral acid is 
employed, may attack some of the non-calcareous constituents, so that it cannot be 
concluded that the residue al»solutely represent* ever}'thing present in the rock except 
the carbonate of lime ; but the projwrtion of non-calcareoiw matter so dissolved by the 
acid will usually be small. 

Further cheinical processes. — A thorough chemical analysis of a rock or mineral is 
indispensable for the elucidation of its comi)Osition. But there are several i>roces8es by 
which, until that complete analysis has l>een made, the geologist may add to his know- 
ledge of the chemical nature of the objects of his study. It is commonly the case that 
minerals about which he may be doubtful are ]>reci8ely those which, from their small 
size, are most difficult of separation fi"om the rest of the rock preparatory to analytical 
processes. The mineral apatite, for exam})le, occurs in minute hexagonal prisms, which 
on cross-fracture might l:)e mistaken for nepheline, or even sometimes for quartz. If, 
however, a drop of nitric acid solution of molyMate of ammonia be placed upon one of 
these crystals, a yellow precipitate will api)ear if it I ►e aimtit^;. Xepheline, which is 
another hexagonal mineral likewise abundant in some rocks, gives no yellow precipitate 
with the ammonia solution, while if a drop of hydrochloric acid l)e jnit over it, crystals 
of chloride of sodiimi or common salt will be obtained. These reactions can l)e observed 
even with minute ci-ystals or fragments, by placing them on a glass slide under the 
microscoiKJ and using an exceedingly attenuated pipette for dropping the liquid on 
the slide. ^ 

Two ingenious api)lications of chemical processes to the detennination of minute 
fragments of minerals are now in use. In one of these, devised by Boricky,* 
hydrofluosilicic acid of extreme purity Is employed. This acid decomi)ose8 most 
silicates, and forms from their bases hydrofluosilicates. A i)article about the size of a 
pin's head of the mineral to be examined is fixed by its base u^jon a thin layer of Canada 
balsam spread upon a slip of glass, and a drop of the acid is placed upon it. The 
preparation is then set in moist air nrar a saucer of water under a bell-glass for twenty- 
four houi*s, after which it is enclosed in dry air, with chloride of calcium. In a few 
hours the hydrofluosilicates ciystallize out upon the balsam and can be examined with 
the microscope. Those of jiotassium take the form of cubes, of sodiimi hexagonal 
prisms, Ac. 

The second process, devised by SzaIk), consists in utilizing the colorations given to 
the flame of a Bunsen-burner by sodium and jjotassium. An elongated splinter of the 
mineral to be examined is fii-st placed in the outer or oxidizing part of the flame near 
the base, and then in the reducing }»art fui-ther up and nearer the centre. The amount 
of sodium i>resent in the mineral is indicated by the extent to which the flame is coloured 
yellow. The potassium is similarly estimated, but the flame is then looked at with 
cobalt glass, so as to eliminate the influence of the sotlium.^ 

Blow-pipe Testa. — The chemical tests with the blow-pipe are simple, easily applied, 
and require only patience and practice to give gioat assistance in the detennination of 
minerals. If unacquainterl with blow-piin^ analysis, the student nmst refer to one or 

^ An excellent treatise on the chemical examination of minerals under the microsco])e is 
that by MM. Klement and Renard, * Inactions microchemiques fi cristaux et lenr applica- 
tion en analyse qualitative,' Brussels, 1886. See also H. Behrens, Ann. icoU Polytechnique 
de Del/tf i. ISSfi p. 176 ; Xeucit Johrh. vii. Beilage Baud. p. 43.') ; Zvitsch./. Analyt. Chfmie, 
XXX. ii. p. 126-174 (1891). 

'^ Archiv Natvnmss. Ltnidesifyrch/nracfntiifj rou Bohmen, iii. fasc. 3, 1876. 'Elemente 
einer ueueu cheniisch-niikroskoi^isclien Mineral- und Gesteinsanalyse.' Prag, 1877. 

' Szabo, * Ueber eiue neue Methodic die Felspathe audi in Gesteinen zu bestimmen.' 
Buda-Pest, 1876. 



»ART II § iii DETERMINATION OF ROCKS 89 

•ther of the numerous text-books on the subject, some of which are mentioned below. ^ 
•"or early practice the following apparatus will be found sufficient : — 

1. Blow-pipe. 

2. Thick-wicked candle, or a tin box filled with the material of Child's night-lights, 
»nd ftiniished with a piece of Freyberg wick in a metallic support. 

8. Platinum-tipped forceps. 

4. A few pieces of platinum wire in lengths of three or four inches. 

5. A few pieces of platinum foil. 

6. Some pieces of charcoal. 

7. A number of closed and open tubes of hard glass. 

8. Three small stoppered bottles containing sodium -carbonate, borax, and micro- 
cosmic salt. 

9. Magnet. 

This list can be increased as experience is gained. The whole apparatus may easily 
le packed into a box which will go into the corner of a ^portmanteau. 

iii. Chemual Synthesis, 

As already remarked (j). 64), much interesting light has been thrown on the natural 
t>nditions in which minerals and rocks have been formed, by actual experiments in 
rhich these bodies are reproduced artificially. Since the classic experiments of Hall 
auch progress has been made in this subject, notably from the prolonged and admirable 
'esestxihes carried on in Paris by Professor Daubree and by Messrs. Fouqu6 and Michel- 
'j^ry. To some of the results obtained by these observers reference will be made in 
look III. Part I. Sect. iv. The j>roces8es of investigation have been grouj)ed 
n three classes. 1st. Those by the * dry way ' as in fusion and sublimation, sometimes 
imply, sometimes with the intervention of a mineralizing agent such as borax, borates, 
luorides, chlorides, &c. 2nd. Those by the * wet way ' where water or steam are used 
is dissolvauts either by tliemselves or with the aid of some mineralizing agent, and 3rd, 
rhose where some combination of the two foregoing methods is employed, that is, where 
rater or steam is made to act at a high tenii^erature and imder great pressure.* 

iv. Mi^nrascopic Investigation.^ 

The value of the microscoj^ as an aid in geological research is now everywhere 
cknowledged. Some infonnation may here be given as to the methods of procedure in 
nicroscopical inquiry. 

^ Tlie great work on the blow-pipe Is Plattner's, of which an English translation has been 
»iiblishe<l. Elderhorst's 'Manual of Qualitative Blow -pipe Analysis and Determinative 
fineralog>',* by H. B. Nason and C. F. Chandler (Philadelphia : N. S. Porter and Coates), 
I a smaller but useful volume ; while still less pretending is Scheerer's * Introduction to the 
Jse of the Mouth Blow-pipe,' of which a third edition by H. F. Blandford was published in 
876 by F. Norgate. An admirable work of reference will be found in Professor Brush's 
Manual of Determinative Mineralogy ' (New York : J. Wiley and Son). F. v. Kobell's 
Tafein znr Bestimmung der Mineralieu ' (Munich) are useful. A valuable summary will be 
oand in Prof. Cole's 'Aids in Practical Geology,' 1891. 

• See on this subject Daubree's great work 'Geologie Experiraentale,' 1879 ; Fouque et 
€ichcl-Levy, *Synthese des Mineraux et des Roches,' 1882; Stanislas Meunier, * Les 
l^thodes de Syuth^ en Mineralogie, ' 1891 ; ti\so postea, p. 302 et seq. 

• On the microscopic investigation of rocks consult Fouque and Michel-Levy, * Min^ralogie 
tficrogfaphique,' 2 vols. Paris, 1879 ; Michel-Levy, * Les Min«^raux des Roches,' Paris, 1888 ; 
lichcl-L^vyand Lacroix, 'Tableaux des Mineraux des Roches,' 1889 ; Rosenbusch, * Mikro- 
kopische Physiographie der Mineralien und Gesteine,' 2 vols., one of which has been trans- 
ited into English by Iddings and published by Macmillan and Co. ; also his * Hiilfstabellen 



90 GEOGXOSY book n 

1. PreparatiGn of mioroBCopic slides of rocks and mlnarals. — Tlie ohserver 

ought to l>e able to ]>re))aro his own slices, and in many cases will Hnd it of advantage 
to do so, or at least personally to Aui)erintend their preparation by others. It is 
desirable that he should know at the outset that no costly or unwieldy set of apparatus 
is needful for his purpose. If he is resident in one place and can accommodate a catting 
machine, such as a lapidary's lath, he Mill find the process of preparing rock-slices 
greatly facilitated.^ The thickness of each slice must be mainly regulated by the nature 
of the rock, the rule being to make the slice as thin as can conveniently be cut, so as to 
save labour in giindiug down afterwards. Perhaps the thickness of a shilling may be 
taken as a fair average. The operator, however, may still further reduce tliis thicknesi 
by cutting and polishing a face of the si>ecimen, cementing that on glass in the way to 
1>e immediately de8crilx^d, and then cutting as close as possible to the cemented sur&oe. 
The thin slice thus left on the glass can then be ground down with comjiarative ease. 

Excellent rock -sections, however, may be prepared without any machine, provided 
the operator possesses ordinary neatness of hand and patience. He must procure as thin 
chii»s as ])ossible. Should the rocks be accessible to him in the field, he should select 
the freshest portions of them, and by a dexterous use of the hammer, break off from a 
sharp edge a number of thin splinters or chips, out of which he can choose one or m<at 
for rock-slices. These chips may l>e about an inch s(|uare. It is well to take several of 
them, as the first s])ecimen may chance to be s]K)iled in the preparation. The geologist 
ought also always to carry off a ])i(^e of the same block from which his chip is taken, 
that he may have a s|>ecimen of the rock for future reference and comparison. Eveiy 
such hand-specimen, as well as the chips 1)elonging to it, ought to be wrapped up in 
I>aper on the s]K>t where it is obtained, and with it should be ]>laced a label containing 
the name of the locality and any notes that may l>e thought necessary. It can hardly 
be too fre^^uently reiterated that all such field-notes ought as far as }H)ssible to be written 
down on the ground, when the actual facts are before the eye for examination. 

Having obtained hia thin slices, either by having them slit with a machine or by 
detaching with a hammer as thin splinters as |)ossible, the o])erator may proceed to the 
prc])aration of them for the microscop. For this puriH)8e the following simple apparatus 
is all that is absolutely needful, though if a grinding-machine be added it will save 
time and labour. 

List of Apparatus required in the Prcimratian of Thin Slices of Rocks and AfineraU 

for Microscopical Examination. 

1 . A cast-iron plate \ inch thick and 9 inches square. 

2. Two jueccs of plate-glass, 9 inches s<[uare. 

zur Mikroskopischen Mineralbestimmung,' 1888, translated into English by F. H. Hatch 
and published by Swan Sonnenschein k Co. ; F. Uutley, ' Kock-foruiiug Minerals,' London, 
1888, and Prof. Cole's volume above cited. 

^ A machine well adapted for both cutting and polishing was devhted some years ago by 
Mr. J. B. Jordan, and may be had of Messrs. Cotton and Johnson, Gerrard Street, Soho, 
London, for £10, 10^. Another slicing and polishing machine, invented by Mr. F. 6. Cnttell, 
costs £6, 105. These machines are too un\\ieldy to be carried about the country by a field- 
geologist. Fuess of Berlin 8ui>plies two small and convenient hand-instruments, one for 
slicing, the other for grinding and polishing. The sliciug-machine Is not quite so satisfactory 
for hard rocks as one of the larger, more solid forms of apparatus worked by a treadle. Bat 
the grinding-machine is useful, and might be added to a geologist's outfit without material 
inconvenience. If a lapidary is within reach, much of the more irksome part of the work 
may be saved by getting him to cut off the thin slices in directions marked for him upon the 
specimens. Many lapidaries now undertake the whole labour of cutting and monntiiig 
microscopic slides. 



ART II § iii DETERMINATION OF ROCKS 91 

8. A Water of Ayr stone, 6 inches long by 2} inches broad. 

4. Coarse emery (1 lb. or so at a time). 

5. Fine or flour-emery (ditto). 

6. Putty powder (1 oz.) 

7. Canada balsam. (There is an excellent kind prepared by Rimmington, Bradford, 
peeially for microecopio preparations, and sold in shilling bottles. ) 

8. A small forceps, and a common sewing-needle with its head fixed in a cork. 

9. Some oblong pieces of common flat window-glass ; 2 x 1 inches is a convenient 
lie. 

10. Glasses with ground edges for mounting the slices upon. They may be had 
.t any chemical instrument maker's in different sizes, the commonest in this country 
mng 3x1 inches, though this size is rather too long for convenient handling on 
k rotating stage. 

11. Thin covering-glasses, square or round. These are sold by the ounce ; 4 oz. will 
m saiBcient to begin with. 

12. A small bottle of spirits of wine. 

The first part of the process consists in rubbing down and polishing one side of the 
hip or slice, if this has not already been done in cutting off" a slice affixed to glass, 
• above mentioned. We place the chip upon the wheel of the grinding-inachine, or, 
iOing that, upon the iron plate, with a little coarse emery and water. If the chip is 
shaped that it can be conveniently pressed by the finger against the plate and kept 
here in regular horizontal movement, we may proceed at once to nib it down. If, how- 
rer, we find a difficulty, from its small size or otherwise, in holding the chip, one side 
f it may be fastened to the end of a bobbin or other convenient bit of wood by means of 
cement formed of three-parts of resin and one of beeswax, which is easily softened by 
leating. A little practice will show that a slow, equable motion with a certain steady 
ttmaie is most effectual in producing the desired flatness of surface. When all the 
(nighnesses have been removed, which can be told after the chip has been dipped in 
rater so as to remove the mud and emery, we place the specimen upon the square of 
late-glass, and with flour-emery and water continue to rub it down until all the scratches 
itieed by the coarse emery have been removed and a smooth polished surface has been 
roduced.^ Care should be taken to wash the chip entirely free of any grains of coarse 
mery before the polishing on glass is begun. It is desirable also to reserve the glass for 
oliahing only. The emery gets finer and finer the longer it is used, so that by remain- 
ig on the plate it may be used many times in succession. Of course the glass itself is 
^om down, but by using alternately every portion of its surface and on both sides, one 
late may be made to last a considerable time. If after drying and examining it carefully, 
'e find the surface of the chip to be polished and ftee from scratches, we may advance to 
lie next part of the process. But it will often ha])pen that the surface is still finely 
sratched. In this case we may place the chip upon the Water of Ayr stone and with a 
ttle water gently rub it to and fro. It should be held quite flat The Water of Ayr 
^ne, too, should not be allowed to get worn into a hollow, but should also be kept quite 
at, otherwise we shall lose part of the chip. Some soft rocks, however, will not take an 
Dscratched surface even with the Water of Ayr stone. These may be finished with 
atty powder, applied with a bit of woollen rag. 

The desired flatness and polish having been secured, and all trace of scratches and 
irt having been completely removed, we proceed to a further stage, which consists in 



1 Exceedingly impalpable emery powder may be obtained by stirring some of the finest 
ncry in water, and after the coarse particles have subsided, pouring ofl' the liquid and 
llowing the fine suspended dust gradually to subside. Filtered and dried, the residue can 
I kept for the more delicate parts of the polishing. 



92 GEOGXOSY book ii 

grinding down tlie opposite side and reducing the chip to the re(|nisite degree of thin- 
ness. The first step is now to cement the ^)olislied surface of the chip to one of the 
pieces of common glass. A thin piece of iron (a common shovel does quite well) is 
heated over a lire, or is placed between two supjKjrts over a gas-flame.^ On this plate 
must be laid the piece of glass to which the slice is to be affixed, together with the slice 
itself. A little Canada balsam is drop{>ed on the centre of the glass and allowed to 
remain until it has acquired the necessary consistency. To test this condition, the point 
of a knife should be inserted into the balsam, and on being removed should be rapidly 
cooled by being pressed against some cold surface. If it soon becomes hard enough to 
resist the pressure of the finger nail, it has been sufficiently heated. Care, however, 
must l)e observed not to let it remain too long on the hot plate ; for it will then become 
brittle and start from the glass at some future stage, or at least will break away from 
the edges of the chip and lo^ve them exposed to the risk of being frayed off. The heat 
should be kept as moderate as {)ossible, for if it becomes too great it may injure some 
portions of the rock. Chlorite, for example, is rendered quite opaque if the heat ia so 
great as to drive off its water. 

When the balsam is found to be I'eady, the chip, which has l^een warmed on the 
same plate, is lifted with the forceps, and laid gently down uiK)n the balsam. It is 
well to let one end touch the balsam fii-st, and then gi*adually to lower the other, as in 
this way the air is driven out. With the jwint of a needle or a knife the chip should 
be moved about a little, so as to ex])cl any bubbles of air and promote a firm cohesion 
luitween the glass and the stone. The glass is now removed with the forceps from the 
plate and put upon the table, and a lead weight or other small heavy object is placed 
upon the chip, so as to keep it pressed down until the balsam has cooled and hardened. 
If the o]>eration ha.s l>ecn successful, the slide ought to be ready for further treatment as 
soon as the balsam has become cold. If, however, the balsam is still soft, the glass must 
be again placed on the plate and gently heated, until on cooling, the balsam fulfils the 
condition of resisting the pressure of the finger-nail. 

Having now produced a firm union of the chip and the glass, we pi"oceed to rub down 
the remaining side of the stone with coarse emery on the iron plat« as before. If the 
glass cannot be held in the hand or moved by the simple ju-essui-e of the fingers, which 
usually suffices, it may be fastened to the end of tlu; bobbin with the cement as before. 
When the chip has been reduced until it is tolerably thin ; until, for example, light 
Hpi)eai*s through it when held l)etween the eye and the window, we may, as l>cfore, wash 
it clear of the course emery and continue the reduction of it on the glass plate with fine 
emery. Crystalline rocks, such as granite, gneiss, diorite, dolerite, and modem lavas, 
can be thus re<luced to the re({uired thinness on the gloss platt?. Softer rocks may 
re<iuire gentle treatment with the Wat4?r of Ayr stone. 

The last parts of the process are the most delicate of all. We desire to make the 
section as thin as possible, and for that j)ur]K)se continue rubbing until after one final 
attempt we may perhajis find to our dismay that gi*eat iwiit of the slice has disappeared. 
The utmost caution should be ust^d. The slide should be kept as flat as ])Ossible, and 
looked at fre<iuently, that the first indications of disniption may be detected. The 
thinness desirable or attainable depends in gi-eat measure u})on the nature of the rock. 
Transjiarent minerals need not be so much reduce^l as more opaque ones. Some 
minerals, indeed, remain absolutely oi>a([ue to the last, like pyrite, magnetite, and 
ilmenite. 

The slide is now ready for the microscoj)o. It ought always to be examined with 
that instrument at this stage. We can thus see whether it is thin enough, and if any 
chemical tests are require<l they can readily be applied to the exjwsed surface of the 

^ A piece of wire-gauze placed over the Hame, with an interval of an inch or more 
between it and the overlying thin iron plate, tends to diffuse the heat and prevent the 
balsam from being unequally heated. 



IT n § iii DETERMIXA TION OF ROCKS 93 

». If the rock has proved to be very brittle, and we have only succeeded in procur- 
a thin slice after much labour and several failures, nothing further should be done 
h the preparation, unless to cover it with glass, as will be immediately explained, 
eh not only protects it, but adds to its transparency. But where the slice is not so 
^e, and will bear removal from its original rough scratched piece of glass, it should 
trmnsferred to one of the glass-slides (No. 10). For this purpose, the preparation is 
e more placed on the warm iron plate, and close alongside of it is ]mt one of the 
of glass which has been carefully cleaned, and on the middle of which a little 
balsam has been dropped. The heat gradually loosens the cohesion of the slice, 
ich is then very gently pushed with the needle or knife along to the contiguous 
in slip of glass. Considerable practice is needed in this pai-t of the work, as the 
e, being so thin, is apt to go to pieces in being transferred. A gentle inclination of 
warm plate, so that a tendency may be given to the slice to slip downwards of itself 
to the clean glass, may be advantageously given. We must never attempt to lift 
•lice. All shifting of its position should be performed with the point of the needle 
>ther shar|> instrument. If it goes to pieces we may yet be able to pilot the frag- 
its to their resting-place on the balsam of the new glass, and the resulting slide may 
mfficient for the required ])urpose. 

When the slice has been safely conducted to the centre of the glass slip, we put a 
le Canada balsam over it, and warm it as before. Then taking one of the thin cover- 
Mes with the forceps, we allow it gradually to rest upon the slice by letting down 
t one side, and then by degrees the whole. A few gentle circular movements of the 
er-glass with the point of the needle or forceps may be needed to ensure the total 
ippearance of air-bubbles. When these do not appear, and when, as before, we find 
t the balsam has acquired the proper degree of consistence, the slide containing the 
e is removed, and placed on the table with a small lead weight above it in the same 
f as already described. On becoming quite cold and hard the su])erabundant balsam 
nd the edge of the cover-glass may be scraped off with a knife, and any which still 
lepcs to the glass may be removed with a little spirits of wine. Small labels should 
kept ready for affixing to the slides to mark localities and reference numbers. TIius 
elled, the slide may be put away for future study and comparison. 
The whole process seems i)erhaps a little tedious. But in reality much of it is so 
ehanical, that after the mode of manipulation has been learnt by a little experience, 
rubbing-down may be done while the oi)erator is reading. Thus in the evening, 
en enjoying a pleasant book after his day in the field, he may at the same time, after 
le practice, rub down his rock-chiiw, and thus get over the dnidgery of the oiieration 
lost unconsciously. 

Boxes, with grooved sides or with flat trays for carrying microscopic slides, are sold 
iifferent sizes. Such boxes are most convenient for a travelling equipage, as they go 

small s|>ace, and with the help of a little cotton-wool they hold the glass slides 
oly without risk of breakage. For a final resting-place, a case with shallow trays or 
men in which the slides can lie flat is most convenient. 

2. The Microscope. — Unless the observer proposes to enter into great detail in the 
estigation of the minuter parts of rock - stinicture, he does not require a large 

1 expensive instrument. For most geological purposes, objectives of 2, 1, and I inch 
al length are sufficient. But it is desirable also for sjiecial work, such as the 
estigation of crystallites and inclusions of minerals, to have an objective capable of 
gnif3ring up to 200 or 300 diameters. An instrument with fairly good glasses of 
ise powers, according to the arrangement of object-glasses and eye- pieces, may be had 
some London makers for £5. But for some of the most imi>ortant i>arts of the 
sroscopical study of rocks a rotating stage is requisite, the presence of which 
wssarily adds to the cost of the instrument. One of the best microscopes s]>ecially 
ipted for petrographical research is that devised by Mr. A. Dick, and manufactured by 
lit k Son, of 81 Tottenham Court Road, London, price £18 without objectives. 



94 GEOGNOSY book n 



Among the indisjKjnsable adjuncts are two Nicol-prisms, one (iK)lari2er) to be fitted 
below the stage, tlie other (analyser) most advantageously placed over the eye-piece. A 
([uartz-wedgc is useful in examination i^ith ]>olarized light. A nose-piece for tvo 
objectives, screwed to the foot of the tube, saves time and trouble by enabling the 
observer at once to pass from a low to a high |)ower. The numerous pieces of apiMratos 
necessary for })liysiological work are not needed in the examination of rocks and 
minerals. 

3. Methods of ETaminatJon. — A few hints may be here given for the guidance of 
the student in making his own microscopic obsen'ations, but he must consult some of 
the s{)ecial treatises, mentioned on p. 89, for full details. 

Rtflccted Light. — It is not infrequently desirable to observe with the microscope the 
characters of a rock as an oiMu^ue object. This cannot usually be done with a broken 
fragment of the stone, except of course with very low |K)wers. Hence one of the most 
useful preliminary examinations of a pre]^>arod slice is to place it in the field, and, 
throwing the mirror out of gear, to converge as strong a light uiM)n it as can be had, 
short of bright direct sunlight. The observer can then see some way into the rock and 
observe the relative thicknesses and forms of its constituents. The advantage of this 
method is particularly noticeable in the ease of o]>a(iue minerals. The sulphides and 
iron-oxides so abundant in rocks appear as densely black objects with transmitted light, 
and show only their external form. But by throwing a strong light upon their surface, 
we may often discover not only their distinctive colours, but their characteristic internal 
structure. Titaniferoiis iron is an admirable example of the advantage of this method. 
Seen with transmitted light, that mineral ap^^ars in black, structureless grains or 
oi)at[ue patches, though frc(|ueutly bounded by definite lines and angles. But with 
reflected light, the cleavage and lines of growth of the mineral can then often be clearly 
seen, and what seemed to be uniform black j^tehes are found in many cases to enclose 
bright brassy kernels of pyiite. Magnetite also })rcsents a characteristic blue-black 
colour, which distinguishes it from the other iron-oxides. 

Transmitted Light. — It is, of course, with the light allowed to pass through prepared 
slices that most of the microscopic examination of minerals and rocks is perfonned. A 
little exi>erience will show the learner that, in viewing objects in this way, he may 
obtain somewhat ditterent results from two slices of the same rock according to their 
relative thinness. In the thicker one, a cei-tain mineral or rock, obsidian for example, 
will ap|>ear i)erhai)s brown or almost black, while in the other what is evidently the 
same substance may be pale yellow, green, brown, or almost colourless. Triclinic 
felsi>ars seen in i»olarizcKi light give only a i>ale milky light when extremely thin, but 
present brij;ht chromatic l>ands when somewhat thicker. 

Polarized Light. — By means of polarized light, an exceedingly delicate method of 
investigation is made available. We use l)oth the Nicol- prisms. If the object be singly- 
refracting, such as a piece of glass, or an amorphous body, or a crystal 1)elonging to 
some substance which crystallizes in the isometric or cubic system (or if it \)e a tetragonal, 
hexagonal or rlionibohedral crystal, cut i)ei']»endicular to its princi];»al axis), the light 
will reach our eye apparently unaffected by the intervention of the object. The field 
will remain dark when the axes of the two prisms are at right angles (crossed Nicols), in 
the same way as if no intervening object were thei-e. Such bodies are isotropic^ In 
all other cases, the substance is doubly • refracting and modifies the (solarized beam 
of light. On rotating one of the prisms, we ]^)erceive l)ands or flashes of colour, and 
numerous lines ap{iear which before were invisible. The field no longer remains dark 
when the two Nicol- prisms are crossed. Such a substance is anisotropic. 

It is evident, therefore, that we may readily tell by this means whether or not a 
rock contains any glassy constituent. If it does, then that jiortion of its mass will 
become dark when the prisms are crossed, while thtj ciystallinc i»arts which, in the vast 

' But the effect of pressure may give weak colour-tints in glasses and in cubic crystals. 



ABT II § Hi DETERMINATION OF ROCKS 95 

tugoiity of cases, do not belong to the cubic system, will remain conspicuous by their 
frightness. A thin plate of quartz makes this separation of the glassy and crystalline 
larts of a rock even more satisfactory. It is placed between the Nicol-prisms, which 
nay be so adjusted with reference to it that the field of the microscope appears uniformly 
riolet. The glassy portion of any rock, being singly-refracting or isotropic, placed on 
he stage will allow the yiolet light to pass through unchanged, but the crystalline 
portions, being doubly-refracting or anisotropic, will alter the violet light into other 
[Hismatic colours. The object should be rotated in the field, and the eye should be kept 
iteadily fixed upon one portion of the slide at a time, so that any change may be observed. 
rhiB is an extremely delicate test for the presence of glassy and crystalline constituents. 

In searching for the crystallographic system to which a mineral in a microscopic 
slide should be referred, attention is given to the directions in which the mineral placed 
betwieen crossed Nicols appears dark, or to what are called the directions of its extinc- 
tion. It is extinguished (that is, the normal darkness of the field between the crossed 
NiooIb is restored) when two of its axes of elasticity for vibrations of light coincide with 
the principal sections of the two prisms. During a complete rotation of the slide in the 
field of the microscope the mineral becomes dark in four positions 90"* apart, each of 
vrhich marks that coincidence. When, on the other hand, the prisms are placed parallel 
to each other, the coincidence of their principal sections with the axes of elasticity in 
the mineral allows the maximum of light to pass through, which likewise occurs four 
times in a complete rotation of the mineral. The different crystallographic systems are 
distinguishable by the relation between their crystallographic axes and their axes of 
elsaticity. By noting this relation in the case of any given mineral (and there are 
nsoally sections enough of each mineral in the same rock-slice to furnish the required 
data) its crystalline system may be fixed. But in many cases it has been found possible 
to establish characteristic distinctions for individual mineral species, by noting the 
angle between the direction of their extinction and certain principal faces. 

The determination of whether the component grains of a rock belong to uniaxial or 
biaxial doubly-refracting minerals is a point of much importance, which is effected by 
means of an achromatic condenser inserted in the apei-ture of the stage below the slide 
and soitably adjusted so as to converge the rays of light within the grain or crystal. The 
Kiools having been crossed, the eye-piece is removed, and the eye when held a little 
distance from the open end of the tube will perceive a dark bar, ring, or cross move across 
the field as the stage is rotated, if the mineral examined has been cut at a favourable 
angle. By the form and behaviour of these indications the uniaxial or biaxial character 
is made evident. 

PUoehraism (Dichroism). — Some minerals show a change of colour when a Nicol- 
prism is rotated below them ; hornblende, for example, exhibiting a gi*adatiou from deep 
brown to dark yellow. A mineral presenting this change is said to be pleochroic 
(polychroic, dichroic, trichroic). To ascertain the pleochroism of any mineral we may 
remove the upper polarizing prism (analyser) and leave only the lower (polarizer). If 
as we rotate the latter, no change of tint can be observed, there is no pleochroic mineral 
p r ese nt, or at least none which shows pleochroism at the angle at which it has been 
bisected in the slice. But in a slice of any crystalline rock, crystals may usually be 
observed which offer a change of hue as the prism goes round. These are examples of 
pleochroism. This behavioiu* may be used to detect the mineral constituents of rocks. 
Thus the two minerals hornblende and augite, which in so many respects resemble each 
other, cannot always be distinguished by cleavage angles, in microscopic slices. But as 
Tschermak pointed out, augite remains passive or nearly so as the lower prism is rotated : 
it is not pleochroic, or only very feebly so ; while hornblende, on the other hand, 
especially in its darker varieties, is usually strongly pleochroic. It is to be observed, 
however, that the same mineral is not always er^ually pleochroic, and that the absence 
of this property is therefore less reliable as a negative test, than its presence is as a 
positiye test. 



9 

96 GEOGNOSY book n 



It would be beyond tlie scope of tliis volume to enter into the complicated details of 
the microscopic structure of minerals and rocks. This information must be sought in 
some of the works specially <levoted to it, a few of which are cited on p. 89. 

In his examination of rocks with the microscope, the student may find an advantage 
in pro])ounding to himself the following questions, and i-eferring to the pages here 
cited. 

1st, Is the rock entirely crystalline (pp. 97, 148, 154), consisting solely of crystals of 
different minerals interlaced ; and if so, what are these mineiuls ? 2nd, Is there any trace 
of a glassy ground-mass or base (])p. 100, 114) If Should this \)e detected, the rock is 
certainly of volcanic origin (pp. 162, 171). 3rd, Can any c\'idence be found of the devitri- 
tication of wliat may have l>een ut one time the glassy basis of the whole rock ? This 
devitrification might be shown by the appearance of numerous microscopic hairs, rods, 
bundles of feather-like in-egular or gi*anular aggregations (p. 115). 4th, In what uitier did 
the minerals crystallize ? This may often be matle out with a niicrosco])e, as, for instance, 
where one mineral is enclosed within another (p. 114).* 5th, What is the nature of any 
altei-ation which the roi'k may have undergone ? In a vast number of cases the slices 
show abundant evidence of such alteration : felsjMir passing into gi-anular kaolin, augite 
changing into viridite, olivine into serpentine, while secondaiy calcite, e])idote, quartz, 
and zeolites run in minute veins or till up interstices of the rock (p. 345). 6th, Is the 
rock a fragniental one ; and if so, what is the nature of its comiH>nent gi*ains (p. 127)1 
Is any trace of organic remains to be detected. 



§ iv. — General outward or Megascopic (Macroscopic) Chaxacters of Bocks.* 

I 

1. Structure.^ — The diifereiit kinds of rock-structures distinguishable 
by the unaided eye are denoted either by ordinary descriptive adjectiveSi 
or by terms derived from rocks in which the special structures are 
characteristically developed, such as gi-anitoid, brecciated, shaly. It 
must be borne in mind, however, that the external character of a rock 
does not always supjdy us with its true internal structure, which may be 
gained only by microscopic examination. This is of course more especially 

* It is i)0ssible, however, that a crystal enclosed within another may sometimes have 
crystallized tliere out of a portion of the surrounding magma of the rock which has been 
unclosed within the larger crystal {^tstea^ p. 303). 

- The following general text-books on rocks may l)e referred to : Macculloch, * A 
Geological Classilication of Rocks,* &c., Loudon, 1821. B. von. Cotta, ' Rocks Classified 
and Described,' translated by Lawrence, Loudon, 1866. Zirkel, * Lehrbuch der Petrographie,' 
two vols. lionn, 1866. Senft, 'Classification der Felsarten,' Breslau, 1857; *.Die 
Krystalliuischen Felsgeiuengtheile,* Berlin, 1868. Kenngott, * Eleniente der Petrographie,' 
Leipz. 1868. A. von Lasaulx, 'Elemente der Petrographie,' Bonn, 1875. Bischof, 
* Chemical Geology,' translated for Cavendish Society, 1854-59, au<l supplement, Bonn, 
1871. Roth, * Allgenieine und Chemische Geologic,' Berlin, 18/9. Other works in which 
the niicrascoincal characters are more si>ecially treated of, are enumerated on p. 108. 

'^ In the 3rd e«iition of Jukes' 'Student's Manual of Geology* (1871), p. 93, it wae 
proi>osed to reser\'e the term ** Structure " for large features, such as characterise rock-blockf, 
and to use the term "Texture " for the minuter characters, such as can be judged of in hand 
specimens. M. De Lai)parent makes a similar distinction (Traite, p. 602, note). But the 
practice of using the word structure as it is employed above in the text, has received such a 
support from the petrographers of Germany that though I still think it would be preferable 
to distinguish 1)etweeu texture and stmcturey I have adopted what has now the sanction of 
coimnon usage. 



ART II § iv MEGASCOPIC CHARACTERS OF ROCKS 97 

rue of the close-grained kinds, where to the naked eye no definite 
tructure is discernible. Some of the definitions originally founded on 
xtemal appearance have been considerably modified by microscopic 
fivestigation. Many compact rocks, for instance, have been proved to be 
rhoUy crystalline. 

The same rock-mass may show very different structures and textures 
n different parts of its extent. This is true alike of sedimentary and 
gneous materials. It may be observed even in the several portions of 
►ne continuous mass of erupted rock — variations in the rate of cooling, in 
emperature, and other circumstances have combined to produce some- 
imes the most extraordinary textural and even structural, as well as 
hemical and mineralogical contrasts in a boss or sheet of igneous 
ock.^ Hence the student must be on his guard against concluding that 
wo portions of rock strikingly unlike each other in outward appearance 
annot be portions of one original continuous mass. 

Crystalline (Phanerocrystalline), consisting wholly or chiefly 
I crystalline particles or crystals.^ Where the individual elements of 
he rocks are of large size, the structure is coarse-crystalline (granitic), as in 
aany granites. When the particles are readily visible to the naked eye, 
nd are tolerably uniform in size, as in marble, many granites and 
domites, the rock is said to be granular-crystalline. Successive stages 
a the diminution of the size of the particles may be traced until these 
re no longer recognisable with the naked eye, and the structure must 
hen be resolved with the microscope {fine-crystalline^ micro-ci'ystalline, crypto- 
rydaUine). Fine-grained rocks may also be called compad, though this 
erm is likewise applicable to the more close-grained varieties of the 
ragmental series. The microscopic characters of such rocks should 
Iways be ascertained where possible.^ 

Many crystalline rocks consist not only of crystals, but of a magma 
r paste, in which the crystalline particles are seen by the naked eye to 
■e imbedded. It is of course impossible, except from analogy, to deter- 
line macroscopically what may be the nature of this magma. It may 
e entirely composed of minute crystals, or may consist of various 
rystallitic products of devitrification. Its intimate structure can only 
e ascertained with the microscope. But its existence is often strikingly 
lanifest even to the unassisted eye, for in what are termed " porphyries " 
; forms a large part of their mass. The term ^^ ground-rnass ^^ is 
mployed to denote this megascopic matrix. Microscopic examination 
hows that a ground-mass may consist of minute crystals, or crystallites, 
r granules and filaments, or glass, or combinations of these in various 
roportions. (See pp. 109, 117.) 

Lithoid, compact and stony in aspect, with no very distinct crystalline 

* See postetty pp. 564, 576 ; G. F. Becker, Amer. Joum, Sci. xxxiii. (1887), p. 50. J. 
L L. Vogt, Oe^l. Foren, Forhand. Stocklwlniy xiii. (1891). 

' Prof. Rosenbusch proposed the terni holocry stall ine for rocks in which there is uo 
orphous material among the crystalline constituents. 

• On the crystaUization of igneous rocks, see J. P. Idclings, Bull. Phil. Soc. Washington, 
. (1889), p. 71. 

H 



98 GEOGNOSY book ii 

structure. The term is especially applied to the devitrified condition of 
once glassy rocks, such as obsidians, which have asaumed the character oE 
perlitos or felsites. 

Granitic (Granitoid), thoroughly crystalline, and consiating of 
crystals approximately uniform in size, as in granite. This s^ucture 
in characteristic of many eruptive rocks. Though usually distinctly 
recognisable by the naked eye (" macromerito " of V<^elsang '^ it 
sometimes becomes very fine (" micronierite "), and may be only 
recognisable with the microscofw as thoroughly crystalline (micrognn- 
itic) ; at other times it pmfses into a porphyritic or porphyroid chaiwster 
by the ap|>earance of largo crystals di»>persed through a general 
ground-mass. 

Pegmat{tic(Pegmatoid, Graphic), exhibiting the peculiar arrange- 
ment of crystalline constituents seen in i>egmatite or graphic granite 
(p. 158), where the quartz and felspu 
have crystallized simultaneously so 
i\s to be enclosed within each other. 
This structure may bo seen on a large 
scale in many massive veins of peg- 
matite ; where it takes an exceedingly 
minute form it is known as micro- 
pegmatitic (Fig. 5). Such micro- 
scopic intergrowth of quartz and fel- 
spar is characteristic of large maaaet 
of eniptive rock (micropegmatite, 
gi-anophyre). 

Aphanilic, a name given to tJie 
veiy close texture exhibited by some 
igneous rocks {diabases, diorites) where 
the component ingredients cannot be 
deteiTuined except with the microscope. 
Porphyritic (Porphyroid), composed of a compact or finely 
crystalline ground-mass, through which larger crystals of earlier con- 
solidation,- often of felspar, are dispersed (Fig. G). This and the granitic 
structure are the two great structure* types of the eruptive rocks. 
By far the largest number of these rocks belong to the porphyritic 
type. Microscopic research has thrown much light on the nature of 
the ground-mass of porphyritic rocks. Vogelsang proposed to clasBify 
these rocks in three divisions : ' 1st, Grwnophyre, where the ground-mass, 
is a microscopic crystalline mixture of tbe component minerals with 
absence or sparing development of an imperfectly individualised magma 
(seep. 118); 2nd, ft/s(>p/iyre, having usually an impei-fectly individualised 
or felsitic magma for the ground-mass (pp. 117, 119); 3rd, VUrophyre, 
where the ground-mass is a glassy magma (pp. 114, 120), The second 

' Z. DaUacJi. Qtol. Qea. iiiv. p. 534. 

' PtienocTfstB, Iddiuga, Bull. Phil. Soc. Waahinglm, ii. (1889), p. TS. 

' Vogelssng, loc. eit. Compnn the cluwlficfttion into graniloiJ sod iradiyloid, p. 1S5. 




PABT II § iv MEOASGOPIC CHARACTERS OF ROCKS 99 

sub-division embraces most of the porphyries, and a very large number 
of eruptive rocks of all ages.' 






Klg, 6, 



-Puridi^tic Strucli 



Segregated. — In granite and other crystnllino massive rocks, 
vein-like portions, coarser (or finer) in texture than the rest of the mass, 
may be observed. These belong to the last phase of consolidation, when 
segregations from the original molten or viscous magma took place along 
certain lines or round particular centres, where the individual minerals 
crystaUized out from the general mass. They have been sometimes 
termed "segregation," or "exudation" veins. They arc to be dis- 
tinguished from the veins, usually of finer and more acid material, which 
runify through a mass of igneous rock and probably represent portions 
of the original molten magma which remained still liquid and were 
injected into rents of the already consolidated parts. These are the true 
" contemporaneous veins " (p. 580). 

Granular — a somewhat vague term applied to rocks composed of 
approximately equal grains, which are sometimes worn fragments, as in 
MUidstone, Bometimes crystalline particles, as in granite and marble. This 
texture may become so fine as to pass insensibly into compact.' The 
peculiar granular structure found so abundantly among metamorphic rocks 
which have been intensely crushed and in which there seems to have 
been a process of re-crystallization among the powdered particles, has 
been termed granulitic (p. 119). This word, however, is liable to the 
objection that in Germany it is applied to rocks bearing that structure 
while in France it is used for a holocrystalline granite.' 

' Aecording to Rownbusch the porphyritic mRS«ire rocke ue those in which, during the 
diflareot ttign of their producttan, the ibidb minerala have been formed mare than once. 
SnfJaiui. 1SS2(U.}, p. 14. 

' Ai epplied to miMive (8nipti?e) rocki, Rosenbusch would restrict the term granular to 
tbOH in which och Indiridiut constituent sepanted out during but one deSnite atige o( tbe 
prooew of rock-baiJdiiig. Loc. nil. 

* Ulchel-Lirjr, Amt. da Mines, vui. (1875), p. 3S7 ; 'Structure et Classification des 
Bocha BroptiTai,' 1SS9, p. 14. 



100 GEOGNOSY book ii 

Vitroous or glassy, having a structure like that of artificial glass, 
as in obsidian. Among tho crystalline rocks there is often present a 
variable amount of an amorphous ground mass, which may increase until 
it forma the main part of the substance. Tho nature of this amorphous 
portion is described at pp. 114, 130. Its most obvious megascopic con- 
dition is that of a volcanic glass. Most vitreous rocks present, even to 
the naked eye, dispersed grains, crystals, or other enclosures. Under the 
microscope, they arc found to \>e often crowded with minut« crystals and 
imperfect or incipient crystalline forms (pp. 109, 115). Resinous is the 
term applied to vitreous rocks having the lustre of pitchstone, and to 
others which are still less vitreous. Devitrification is the conversion of 
tho vitreous into a crystalline or lithoid structure (pp. 116, 121). 

Streaked, arranged in streaky inconstant lines (Germ. Sehlieren), 
cither parallel or convergent, and often undulating. This structure, 
conspicuously shown by the lines of flow in ritreous rocks (flow-Btructure, 
rtuxion-stnicturo, fluidal- structure) is less marked where the materials 
have assumed definite crystalline forms. It can be seen on a minute scale, 
however, in many crystalline masses when examined tvith the microBcope 
{p. 120). 

Banded, arranged in parallel bands, distinguished from each other 
by colour, texture, structure or composition ; characteristic of many 
gneisses, and of jaspers, flints, hallefiintas and other flinty rocks. This 
term may frequently bo applied to the flow-stnictiu-e of igneous rocks 
referred to in the previous paragraph, likewise to the segregation veins 
of eruptive bosses and sheets, and to the parallel arrangement of materials 
produced in rocka which liavo under intense mechanical pressure been 
cnishe<I and sheared. With the naked eye it is often hardly possible to 
distinguish between the banded stnicture of deritrified igneous rocks and 
that resulting from intense mecbaniciil deformation. 

Mylonitic, a terra inti-oduced to denote the peculiar granular 
stmcture of rocks which liave luidergone intense crushing. The materials 
have been rctluced to minute grnins which have not recrystallized as 
they have done in the graiiulitic structure, llany remarkable examples 
of this stnicture have been observed 
among the schists of the Scottish High- 
lands. 

Spherulitic, eomixised of, or con- 
taining small globides or spherules which 
may Ih' colloid and isotropic, or more or 
less distinctly crystalline, [urticularly 
with an internal tibi-ous divei^ent struc- 
ture (Figs, r, 17). This structure occurs 
in vitreous mcks, where it is one of 
the stages of devitrification in obsidian, 
pit<.-hstonc, etc' (jx 121). 

The term lithophvse has been 
.:--!'i.iir™ii;icsmi«i.n-. i>i.vniar.i.) ;,j)j,iiod by F. von Kichthofen to brge 
' Od th« coniitlliitioi) anil ot^n of splietnlit* in sciil «niptiTF rocka, wc Vhitauit. 




UEOASCOPIC CHARACTERS OF BOCKS 



101 



bladder-like apherulitea wherein interspaces lined with crystals occur 
between the auccessive concentric internal layers.^ Many andont rhy- 
olites present an aggregate of nodular bodies (Pyromeride) duS originally 
to devitrification and subsequently more or less altered esp^ciaily by 
the deposition of silica within them (poslea, p. 161). 

Orbicular structure is one in which the component minerals of an>Ck 
have crystallized in such a way as to form spheroidal aggregations 'some- 




times with an internal radial or concentric grouping. It is typically s 
in the napoleonite or ball-diorite (kugel- 
diorit«, orbicular diorite, p. 165) of Cor- 
sica (Fig. 8), but occurs in other rocks, 
sometimes even in granite. 

Perlitic (Figs. 9 and 20), having 
the structure of the rock formerly termed 
perlite, wherein between minute rectili- 
near fissures the substance of the mass ha^ 
assumed, during the contraction resulting 
from cooling a finely globular character, 
not unlike the spheroidal structure seen 
in weathered basalt which is also a phe- 
nomenon of contraction during the cool- 
ing and consolidation of an igneous rock. 

Crom, Ptiil. Sic. Waihingtim, xi. p. Ill (18S1) d J P. Iddiugs, o}>. cil. p, 446. 
Qoirtz ummca In «oioe rocks (e.g. baud J ntea) ti 1) globular structure which was 
developed before the ceBution of the m t a th t prod ed How.atructure, and which, 
according to M. Michel-Levy, may be reg led as t g the colloid and cryelalliied 

conditiaoi of silica. Ball. Soc, Otot. Fra u (3) p 140 

' JaKri. K. K. OeoL Meickianil, 1880 p 180 See Idd g», 7lh Ana. Rep. U.fi. Otal. 
Surv. (18SG-SS), p. 249. Amer. Jmm. &i (1887) p. 38. 




102 ;. '-■ flEOGNOSY Booin 

Horny,-(irnty, having a compact, homogeneous, dull texture, like 
that of tfohi or flint, as in chalcodony, jasper, flint, and many halleflintaa 
and iol^it^i. 

O^yitrnoMB (porous), containing irregular cavities due, in moat 
caac^-to the abstraction of some of the minerals ; but occasionally, as in 
some limCBtoneB (sinters), dolomites and lavas, forming part of the 
(irigjrial structure of the rock. 
.'■/-[■Cellnlar. — Many lavas, ancient and modern, have been saturated 
'^•.'mth steam at the time of their eruption, and in consequence of the 
-segregation and expansion of this imprisoned va]K>ur, have had spherical 
cavities developed in their mass. When this cellular structure is marked 
by comparatively few and small holes, it may be called vesicular j wh^v 
the rock consists partly of a roughly cellular, and partly of a more 
compact substance intermingled, as in the sli^ of an iron furnace, it ia 
said to bo slaggy ; i>ortions where the colls occupy about as much space 
as the solid part, and vary much in size and shape, are called scoriaceous, 
this being the character of the rough clinker-like scoriie of recent lava- 
streams ; when the cells are so much more numerous than the solid part, 
that the stone would almost or tiuite float on water, the structure is called 
pumiceouB, the term jntmice being applied to the froth-like part of 



4 f. /^^J^fL^ * 



Fig, lO.-Aiiij^Blui.lnl Stni.tiiiTs ; l'.>r|ili)Titf, old K<-.1 Sanastolw, Armhlre. (K»t. ■1»-) 

obsidian. As the cellular stnicture can only be developed while the rock 
is still liquid, or at least \-iscid, and as, while in this condition, the man 
is often still moving away from its point of emission, the cells are not 
infrequently elongated in the direction of movement Subsequently, 
water infiltrating through the rock, deposits various mineral substances 
(calcite, fpiartz, chalcedony, zeolites, etc.) from solution, so that the 
flattened and elongated nlmond-shaped cells are eventually filled up. 
A cellular rock which has undergone this change ia said to be an 
amygdaloid, or amygdaloidal, and the almond-like kernels u'e known 
as amygdalcs (Fig. 10). AVhere the cells or cavernous spaces of a roA 
are lined with crystals and empty inside they are said to be druses or 
druay cavities. 



PART II § iv MEGASCOPIC CHARACTERS OF ROCKS 103 

Cleaved, having a fissile structure superinduced by pressure and 
known as cleavage (see p. 312, 545). The planes of cleavage are inde- 
pendent of those of bedding, though they may coincide with them. A 
cleaved structure is best seen in fine-grained material, and is typically 
developed in roofing-slate, but it may occur in any compact igneous rock. 

Foliated, consisting of minerals that have crystallized in approxi- 
mately parallel, lenticular, and usually wavy layers or folia. Rocks of 
this Idnd commonly contain layers of mica, or of some equivalent readily 
cleavable mineral, the cleavage-planes of which coincide generally with the 
planes of foliation. Gneiss, mica^chist and talc-schist are characteristic 
examples. So distinctive, indeed, is this structure in schists, that it is 
often spoken of as schistose. In gneiss, it attains its most massive 
form ; in chlorite-schist and some other schists, it becomes so fine as to 
pass into a kind of minutely scaly texture, often only perceptible with the 
microscope, the rock having on the whole a massive structure. 

Fibrous, consisting of one or more minerals composed of distinct 
fibres. Sometimes the fibres are remarkably regular and parallel, as in 
fibrous gypsum, and veins of chrysotile, fibrous aragonite or calcite (satin- 
spar) ; in other instances, they are more tufted and irregular, as in asbestos 
and actinolite-schist. 

Clastic, fragmental, composed of detritus (p. 121). Rocks possess- 
ing this character have, in the great majority of cases, been formed in water, 
and their component fragments are usually more or less rounded or water- 
w<»ni. Different names are applied, according to the form or size of the 
fragments. Brecciated, composed, like a breccia, of angular fragments, 
which may be of any degree of coarseness. Agglomerated, consisting 
of large, roughly rounded and tumultuously • grouped blocks, as in the 
agglomerate filling old volcanic funnels. Conglomerated (Conglo- 
meratic)^ made up of well-rounded blocks or pebbles ; rocks having this 
character have been formed by and deposited in water. Pebbly, 
containing dispersed water-worn pebbles, as in many coarse sandstones, 
which thus by degrees pass into conglomerates. Psamraitic, or sand- 
.stone-Iike, composed of rounded grains, as in ordinary sandstone : when 
the grains are larger (often sharp and somewhat angular) the rock is 
gritty, or a grit. Muddy (pel i tic), having a texture like that of dried 
mud. Cryptoclastic or compact, where the grains are too minute to 
reveal to the naked eye the truly fragmental character of the rock, as in 
fine mudstones and other argillaceous deposits. 

Concretionary, containing, or consisting of mineral matter, which 
has been collected, either from the surrounding rock or from without, 
round some centre, so as to form a nodule or irregularly shaped lump. 
This aggregation of material is of frequent occurrence among water-formed 
rocks, where it may be often observed to have taken place round some 
organic centre, such as a leaf, cone, shell, fish-bone, or other relic of plant 
or animal. (Book IV. Part I.) Among the most frequent minerals found 
in concretionary forms as constituents of rocks, are calcite, siderite, pyrite, 
marcasite, and various forms of silica. In a true concretion, the material 
at the centre has been deposited first, and has increased by additions from 



104 GEOGNOSY book u 

without, either during the formation of the enclosing rock, or by 
subsequent concentration and aggregation. Where, on the other hand, 
ca\'ities and fissures have been filled up by the deposition of materials 
on their walls, and gradual growth inward, the result is known as a 
secretion. Amygdales and the successive coatings of mineral veins arc 
examples of the latter process. 

Septarian — a structui*e often exhibited by concretions of limestone 
and clay -ironstone which in consolidating have shrimk and cracked 
internally. These shrinkagc-ci-acks radiate in an iiTegular way from the 
middle towards the circumference, but die out before reaching the latter 
(Fig. 26). Usually they have been filled with some subsequently infil- 
trated mineral, notably calcite. 

Oolitic, a structure like tish-roe, formed of spherical grains, each of 
which has an internal radiating and concentric structure, and often 
possesses a central nucleus of some foreign l)ody. This structure is 
specially found among limestones (see p. 150). When the grains are 
as large as peas, the structure is termed pi soli tic. 

Various structures which affect large masses of rock rather than 
hand-specimens will be found described in Book IV. But a few of the 
morg imix)rtant may l)e included here. 

Massive, unstratified, ha>ang no arrangement in definite layers or 
strata. Lava, granite, and generally all crystalline rocks which have been 
erupted to the surface, or have solidified below from a state of fusion 
are massive rocks. 

Stratified, bedded, composed of layers or beds lying parallel to 
each other, as in shale, sandstone, limestone, and other rocks w^hich have 
been deposited in water. Successive streams of lava, poured one upon 
another, have also a bedded arrangement. Laminated, consisting of 
fine, leaf -like strata or laminae ; this structure being characteristically 
exhibited in shales, is sometimes also called shalv. 

Jointed, traversed by the divisional planes termed Joints which are 
fully treated of in Book IV. Part II. 

Columnar, divided into prismatic joints or columns. This stnicture 
is typically represented among the basalts and other basic lavas (p. 629 
and Figs. 230-2), but it may also be observed as an effect of contact- 
metamorphism among stratified rocks which have l)een invaded by in- 
trusive masses (p. 599). 

2. Composition. — Before having recourse to chemical or microscopic 
analysis, the geologist can often pronounce as to the general chemical or 
mineralogical nature of a rock. Most of the terms which he employs to 
express his opinion are derived from the names of minerals, and in almost 
all cases are self-explanatory. The following examples may suffice. 
Calcareous, consisting of or containing carbonate of lime. Argilla- 
ceous, consisting of or containing clay. Fel spathic, having some form 
of felspar as a main constituent. Siliceous, formed of or containing 
silica ; usually applied to the chalcedonic forms of this cementing oxide. 
Quart zose, containing or consisting entirely of some form of quartz. 



PART II § iv MEGASCOPIC CHARACTERS OF ROCKS 105 

Carbonaceous, containing coaly matter, and hence usually associated 
with a dark colour. Pyritous, containing diffused disulphide of iron. 
Gypseous, containing layers, nodules, strings or crystals of calcium- 
sulphate. Saliferous, containing beds of, or impregnated with rock- 
salt. Micaceous, full of layers of mica-flakes. 

As rocks are not definite chemical compounds, but mixtures of 
different minerals in varying proportions, they exhibit many intermediate 
varieties. Transitions of this kind are denoted by such phrases as 
** granitic gneiss," that is, a gneiss in which the normal foliated structure 
is nearly merged into the massive structure of granite; "argillaceous 
limestone " — a rock in which the limestone is mixed with clay ; 
" calcareous shale " — ^a fissile rock, consisting of clay with a proportion of 
lime. It is evident that such rocks may graduate so insensibly into each 
other, that no sharp line can be drawn between them either in the field 
or in their terminology. 

As already alluded to, and as will be more fully explained in later 
pages, the progress of research goes to show that even in the same mass of 
eruptive rock considerable differences of chemical composition may be found. 
These differences seem to point to some separation of the constituents, by 
gravity or otherwise, before consolidation. Thus the picrite of Bathgate 
shades upwards into a rock in which the heavy magnesian silicates are 
replaced in large measure by felspars.^ Mr. Iddings has recently called 
attention to some remarkable gradations of composition among the vol- 
canic rocks of the Tewar mountains. New Mexico, where he believes a 
series of intermediate varieties to be traceable from obsidian at the one 
end to basalt at the other.2 A remarkable instance of a similar kind is 
described by Mr. Teall and Mr. Dakyns from the Scottish Highlands. 

3. State of Aggregation. — The hardness or softness of a rock, in 
other words, its induration, friability, or the degree of aggregation of its 
particles, may be either original or acquired. Some rocks (sinters, for 
example) are soft at first and harden by degrees ; the general effect of 
exposure, however, is to loosen the cohesion of the particles of rocks. A 
rock which can easily be scratched with the nail is almost always much 
decomposed, though some chloritic and talcose schists are soft enough to 
be thus affected. Compact rocks which can easily be scratched >\'ith the 
knife, and are apparently not decomposed, may be fine-grained limestones, 
dolomites, ironstones, mudstones, or some other simple rocks. Crystalline 
rocks, except limestone cannot, as a rule, be scratched with the knife 
unless considerable force be used. They are chiefly composed of hard 
silicates, so that when an instance occurs where a fresh specimen can be 
easily scratched, it will usually be found to be a limestone (pp. 82, 139, 
149). The ease with which a rock may be broken is the measure of its 
frangibility. Most rocks break most easily in one direction ; attention 
to this point will sometimes throw light upon their internal structure. 

Fracture is the surface produced when a rock is split or broken, and 

* Trans. Roy. Soc. Edin. vol. xxix. (1879), p. 504. 

« Bull. U. S. Oeol. Surv, No. 66 (1890), Bull. Phil. Soc. Washington, xi. (1890), pp. 
65, 191, tLndpodeat pp. 269, 576. Teall and Dakyns, Qvart. Joum. (itd. Soc. 1892. 



106 GEOGNOSY book n 

depends for its character upon the texture of the mass. Finely granular, 
compact rocks are apt to break with a splintery fracture where wedge- 
shaped plates adhere by their thicker ends to, and lie parallel with the 
general siu'face. When the rock breaks off into concave and convex 
rounded shell-like siuiaces, the fracture is said to be conchoidal, as may 
be seen in obsidian and other vitreous rocks, and in exceedingly compact 
limestones. The fracture may also be foliated, slaty, or shaly, accord- 
ing to the stnicture of the rock. Many opaque, compact rocks are trans- 
lucent on the thin edges of fracture, and afford there, with the aid of a 
lens, a glimpse of their internal composition. A rock is said to be flinty, 
when it is hard, close-grained, and breaks "vnth a smooth or conchoidal 
fracture like flint; friable, when it crumbles down like dry clay or 
chalk; plastic, when, like moist clay, it can be worked into shapes; 
pulverulent, when it falls readily to powder; earthy, when it is de- 
composed into loam or earth ; in coherent or loose, when its particles are 
quite sepanite, as in dry blown sand. 

4. Colour and Lustre. — These characters vary so much, even in the 
same rock, according to the freshness of the surface examined, that they 
possess but a subordinate value. Nevertheless, when cautiously used, 
colour may be made to afford valuable indications as to the probable 
nature and composition of rocks. It is, in this respect, always desirable 
to compare a freshly-broken with a weathered piece of the rock.^ 

//7i?76 indicates usually til e absence or a compai-atively small amount of 
the heavy metallic oxides, esj)ecially iron. It may either be the original 
colour, as in chalk and calc-sinter, or may be developed by weathering, 
as in the whitij crust on flints and on many ix)rphyries. Grey is a fre- 
quent colour of rocks which, if quite pure, would be white, but w^hich 
ac({uire a greyish tint by admixture of dark silicates, organic matter, dif- 
fused pyi'ites, tKrc. Bhw^ or Iduiah-grey is a characteristic tint of rocks 
through which iron-disulphide is diflused in extremely minute subdivision. 
But as a rule it rapidly disa])pcars from such rocks on exposure, especi- 
ally where they contain organic matter also. The stiff blue clay of the 
sea-]>ottoni which is coloured by iron-disulphide becomes reddish-brown 
when dried, and then shows no trace of sulphide.*- Black may ])e due 
either to the presence of carbon (when weathering ^^^ll not change it 
much), or to some iron-oxide (magnetite chiefly), or some silicate rich 
in iron (as hornblende and augit^). Many. rocks (ba.salts and mela^ 
phyrcs particularly) which look quite black on a fresh surface, become 
red, brown or yellow on exposure, black being comparatively seldom a 
weathered colour. Ydlow (or Onnif/e), as a dull earthy colouring matter, 
almost always indicat^^s the presence of hydrated peroxide of iron. In 
modern volcanic districts it may be due to iron-chloride, sulphur, &e. 
Bright, metiillic, gold-like yellow is usually that of iron-disulphide. Brown 
is the normal colour of some carbonaceous rocks (lignite), and ferruginous 
dei)osits (bog-iron-ore, clay-ironstone, S:c.) It very generally, on weathered 

* Alterations of tlie colours of minerals and rocks are effected by lieat and even by gun- 
light. See Jauettaz, Tiull. kSoc. (iwl. xxix. (1872), p. 300. 
' J. Y. Uucliailan, Brit. i4.wc. 1881. p. 584. 



PART n § iv MEGASCOPIC CHARACTERS OF ROCKS 107 

surfaces, points to the oxidation and hydration of minerals containing 
iron. Bed^ in the vast majority of cases, is due to the presence of 
anhydrous peroxide of iron. This mineral gives dark blood-red to 
pale flesh-red tints. As it is liable, however, to hydration, these hues are 
often mixed with the brown, orange and yellow colours of limonite.^ 
Green, as the prevailing tint of rocks, occurs amongst schists, when its 
presence is usually due to some of the hydrous magnesian silicates 
(chlorite, talc, serpentine). It appears also among massive rocks, especi- 
ally those of older geological formations, where hornblende, olivine, or 
other silicates have been altered. Among the sedimentary rocks, it is 
principally due to ferrous silicate (as in glauconite). Carbonate of copper 
colours some rocks emerald- or verdigris-green. The mottled character so 
common among many stratified rocks is frequently traceable to unequal 
weathering, some portions of the iron being more oxidized than others ; 
while some, on the other hand, become deoxidized from the reducing action 
of decaying organic matter, as in the circular green spots so often found 
among red strata. 

Lustre, as an external character of rocks, does not possess the value 
which it has among minerals. In most rocks, the gi'anular texture 
prevents the appearance of any distinct lustre. A completely vitreous 
lustre without a granular texture, is characteristic of volcanic glass. A 
splendent semi-metallic lustre may often be observed upon the foliation 
planes of schistose rocks and upon the laminae of micaceous sandstones. 
As this silvery lustre is almost invariably due to the presence of mica, it 
is commonly called distinctively mimceons. A metallic lustre is met with 
sometimes in beds of anthracite ; more usually its occurrence among rocks 
indicates the presence of metallic oxides or sulphides. A rednous lustre 
is characteristic of many pitchstoncs. Lustre-viotiliiKj is a term applied 
to the intemipted sheen on the cleavage faces of minerals which have en- 
closed much smaller crystals or grains of other minerals. It is well seen 
on the surfaces of some of the constituents of serpentine rocks. 

5. Feel and Smell. — These minor characters are occasionally useful. 
By the feel of a mineral or rock is meant the sensation experienced when 
the fingers are passed across its surface. Thus hydrous magnesian sili- . 
cates have often a marked soapy or greasy feel. Some sericitic mica- 
schists show the same character. Trachyte received its name from its 
characteristic rough or harsh feel. Some rocks adhere to the tongue, a 
quality indicative of their tendency to absorb water. 

SmelL — Many rocks, when freshly broken, emit distinctive odours. 
Those containing volatile hydrocarbons give sometimes an appreciable 
hiiumifums odour, as is the case with certain eruptive rocks, which in 
central Scotland have been intruded through coal-seams and carbon- 
aceous shales. Limestones have often a fetid odour ; rocks full of 
decomposing sulphides are apt to give a s^dphurous odour ; those which 
are highly siliceous yield, on being struck, an empijreumatic odour. It is 
characteristic of argillaceous rocks to emit a strong earthy smell when 
breathed upon. 

1 Sec I. C. Russell, Bull, U. S. Oed. Sure. No. 52 (1889). 



108 GEOGNOSY book n 

6. Specific Gravity. — This is an important character among rocks u 
well as among minerals. It varies from 0*6 among the hydrocarbon 
compK>unds to 3*1 among the basalts. As already stated, the average 
specific gravity of the rocks of the earth's crust may be taken to be about 
2*5, or from that to 3*0. Instruments for taking the specific gravity of 
rocks have been already (p. 85) referred to. 

7. Magnetism is so strongly exhibited by some crystalline rocks as 
powerfully to aflfect the magnetic needle, and to vitiate observations with 
this instrument It is due to the presence of magnetic iron, the existence 
of which may be shown by pulverising the rock in an agate mortar, wash- 
ing carefully the triturated powder, and drying the heavy residue, from 
which grains of magnetite or of titaniferous magnetic iron may be ex- 
tracted with a magnet. This may be done with any basalt (p. 86). A 
freely swinging magnetic needle is of service, as by its attraction or 
repulsion, it affords a delicate test for the presence of even a small quantity 
of magnetic iron. 

§ V. MicroBCopic Characters of Bocks. 

No department of Geology has been more advanced in recent yean 
than Lithology, and this has been mainly due to the introduction of the 
microscope as an instrument for investigating minute internal structure. 
As far back as the year 1827, a method of making thin ti*anspar^t 
sections of fossil wood, and mounting them on glass with Canada balsam, 
had been devised by William Nicol of Edinburgh, and was employed by 
Henry Witham in his * History of Fossil Vegetables.' ^ 

It was not, however, until 1856 that Mr. H. C. Sorby, applying this 
method to the investigation of minerals and rocks, showed how many 
and important were the geological questions on which it was calculated 
to shed light. 2 Reference will be made in subsequent pages to the 
remarkable results then announced by him. To the publication of his 
memoir the subsequent rapid development of the microscopic study (rf 
rocks may be distinctly traced. The microscopic method of analysis 
is now in use in every country where attention is paid to the history 
of rocks.* 

^ Small 4to, Edinburgh, 1831. Tliis work, though dedicated to Nicol, does not distinctly 
recognise him as the actual inventor of the process of slicing mineral substances for micro* 
scopic investigation. All that was original in Witham's researches he owed either directly 
or indirectly to Nicol. 

* Brit. Assoc. 1856, Sect. p. 78. Quart. Jottrn. Geol. Soc. xiv. 1858. Micr. Jaunt. 
xvii. (1877), p. 113. 

' Among the best text-books on this subject the following may be mentioned : — 

* Mikroskopische Beschafienheit der Mineralien und Gesteine,' F. Zirkel, 1 vol. 1873. 

* Mikroskopische Physiographie der Mineralien und Gesteine,' H. Rosenbusch, 2 toIb. 
2nd Edit. 1885-87* and the English translation of the first volume quoted on p. 89 ; likewiae 
the Tables translated by F. H. Hatch quoted on p. 90. * Elemente der Petrographie,' A. 
von Lasaulx, 1875. * Mineralogie micrograph ique : roches eruptives fran^aiaes,' Fonqn^ 
and Michel-Levy, 2 vols. 4to, Paris, 1879. 'Microscopical Petrography,' Zirkel, being toL 
vi. of the Geol. Explor. of 40/A Parallel j Washington, 1876. * British Petrography/ J. J. 
H. Teall, London, 1888. ' Les Mineraux des Roches,' Michel-L^vy and Lacroix, Paris, 1888. 



PART II § V MICROSCOPIC CHARACTERS OF ROCKS 109 

In § iii. p. 90 information has been given regarding the preparation of 
sections of rocks for microscopical examination, the methods of procedure 
in the practice of this part of geological research, and some of the terms 
employed in the following pages. 

1. Microscopic Elements of Rocks. 

Rocks when examined in thin sections with the microscope are found 
to be composed of or to contain various elements, of which the more 
important are, 1st, crystals, or crystalline substances ; 2nd, glass ; 3rd, 
crystallites ; 4th, detritus. 

A. Crystals or Crystalline Substances. — Rock-forming minerals, 
when not amorphous, may be either crystallized in their proper crystal- 
lographic forms (idiomorphic), or while possessing a crystalline internal 
structure, may present no definite external geometrical form (allotriomor- 
phic, p. 1 18). The latter condition is more prevalent, seeing that minerals 
have usually been developed round and against each other, thus mutually 
hindering the assumption of determinate crystallographic contours. 
Other causes of imperfection are fracture by movement in the original 
magma of the rock, and partial solution in that magma (Fig. 1 2), as in the 
corroded quartz of quartz-i>orphyries and rhyolites, and the hornblende 
crystab of basalts. The ferro-magnesian minerals of earlier consolidation 
among basalts and andesites, are sometimes surrounded with a dark shell 
called the corrosion-zone. In some rocks, such as granite, the thoroughly 
crystalline character of the component ingredients is well marked, yet 
they less frequently present the definite isolated crystals so often to be 
obeerved in porphyries and in many old and modern volcanic rocks. 
Among thoroughly crystalline rocks, good crystals of the component 
minerals may be obtained from fissures and cavities in which there has 
been room for their formation. It is in the " dnisy " cavities of granite, 
for example, that the well-defined prisms of felsjmr, quartz, mica, topaz, 
beryl and other minerals are found. Successive stages in order of 
appearance or development can readily be observed among the crystals 
of rocks. Some appear as large, but frequently broken, or corroded 
forms. These have evidently been formed first. Others are smaller but 
abundant, usually unbroken, and often disposed in lines. Others have 
been developed by subsequent alteration within the rock.^ 

A study of the internal structure of crystals throws light not merely 

The Tolnines for the last fifteen or twenty years of the Quarterly Journal of the Geohxjicnl 
Society, Oeoloffieal Magazine^ Neves Jahrbuch fur Mineralogie, <{.r., Zeitschrift der Deutschen 
Oeologischtn Geadlschqftj Bulletin de la SocUti geologique de France^ Jahrlmch der K. K. 
fhohgitchen Reichsanstalt {Vienna), contain numerous papers on the microscopic structure 
of rocka. Rutley's * Study of Rocks,' 1879, and his ' Rock-fomiing Minerals,' 1888 ; Cole's 
•Aids in Practical Geology,' 1891 ; and Hatch's 'Petrology— Igneous Rocks,' 1891, are 
useful handbooks. The manual of Rosenbusch and the work of Fouque and Michel- Levy, 
contain a tolerably ample bibliography of the subject, to which the student is referred. Tlie 
titles of some of the more important memoirs which have recently a])peared will be given in 
footnotes. 

1 Fooqa^ and Michel-L^vy, * Miu. Micrograph.' p. 151. 



1 1 GEOGNOS Y book n 



on their own genesis, but on that of the rocks of which thoy form part, 
and is therefore well worthy of the attention of the geologist. That many 
apparently simple crystals are in reality compound, may not infrequently 
l)e detected by the different condition of weatheiing in the two opposite 
parts of a twin on an exposed face of rock. The internal stinicture of a 
crystal modifies the action of solvents on its exterior {e.g, weathered 
surfaces of calcite, aragonite and felsj)iii*s). Crystals may occasionally be 
observed built up of rudimentiiry ** microlites," as if these were the 
simplest forms in which the molecules of a mineral begin to api>ear (p. 115). 

A microscopic examination of some rocks shows that a subsequent or 
secondary growth of different minerals has taken place after their original 
crysUdline form was complete. These later additions are in optical con- 
tinuity >\'ith the original crystal, and sometimes have taken place even 
upon worn or imperfect forms. They may be occasionally detected 
among the silicates of igneous rocks, and also even among the sandgrains 
of sandstones which have thus had thoii* rounded forms converted into 
crystallographic faces. ^ 

Crystalline minerals are seldom free from extraneous inclusions. 
These are occasionally large enough to be reiulily seen by the naked 
eye. But the microscope reveals them in many minerals in almost 
incredible quiintity. They ai*e, a, vesicles containing gas ; )8, vesicles 
containing liquid ; -y, gloJndes of glass or of some litlioid substance ; 
8, crystals ; €, filaments, or other indefinitely -shaped pieces, patches, or 
streaks of mineral matter. 

a. Gas-filled cavities — are most frequently globular or elliptical, 
and appear to be due to the presence of gas or steam in the crystal at 
the time of consolidation. Zirkel estimates them at 360,000,000 in a 
cubic millimetre of the hauyne from Melfi.^ In some instances the 
cavity has a geometric form belonging to the crystalline system of the 
enclosing mineral. Such a space defined by crystallographic contours is 
a nefjative crystal, A cavity filled with gas contains no bubble, and its 
margin is marked liy a broad dark band. The usual gas is nitrogen, with 
traces of oxygen and carlwn-dioxide ; sometimes it is entirely carbon- 
dioxide or hydrogen and hydrociU'bons. 

p. Vesicles containing liquid (and gas). — As far back as the 
year 1823, Brewster studied the nature of certain fluid-bearing cavities in 
difterent minerals.^ The first observer who showed their important 
bearing on geological researches into the origin of crystalline rocks was 
Mr. Sorby, in whose piper, already cited, they occupy a prominent place. 
Vesicles entirely filled with liquid are distinguished by their sharply- 

^ H. C. Sorby, Presidential Address, Oeol. Soc, 1880, p. 62. R D. Irving and C. R. Van 
Hise *0n secondary enlargements of Mineral Fragments in certain rocks.* Bull, U, 8, OeoL 
Surt\ No. 8 (1884). J. W. Judd. Quart. Journ. Oeol. Soc, xlv. (1889), p. 175. 

2 * Mik. Beschaff.' p. 86. 

^ ISdiju Phil. Journ. ix. p. 94. Trans. Roy. Soc. Edin. x. p. 1. See also W. Niool, 
Edin. Nnc Phil, Journ. (1828), v. \>. 94 ; De la Vallee Pous.«»in and Renard, Aead. Hof. 
Belg. 1876, p. 41 ; Hartley, Journ. Chem. Soc ser. 2, xiv. 137 ; ser. 3, U. p. 241 ; 
Microscop. Journ. xv. p. 170 ; BrU, Assoc, 1877, Sect. p. 232. 



PART II § V MICROSCOPIC CHARACTERS OF ROCKS 111 

defined and narrow black borders. Vesicular spaces containing fluid may 
be noticed in many artificial crystals formed from aqueous solutions 
(crystals of common salt show them well) and in many minerals of 
crystalline rocks. They are exceedingly various in form, being branching, 
curved, oval, or spherical, and sometimes assuming as negative crystals 
a geometric form, like that characteiistic of the mineral in which they 
occur, as cubic in rock-salt and hexagonal in quartz. They also vary 
greatly in size. Occasionally in quartz, sapphire, and other minerals, 
large cavities are readily observable with the naked eye. But they may 
be traced with high magnifying powers down to less than yiriiTiy ^^ ^'^ 
inch in diameter. Their proportion in any one crystal ranges within such 
wide limits, that whereas in some crystals of quartz few may be observed, 
in others they are so minute and abundant that many millions must be 
contained in a cubic inch. The fluid present is usually water, frequently 
with saline solutions, particularly chloride of sodium or of potash, or 
sulphates of potash, soda, or lime. Carbon-dioxide may be present in 
the water ; sometimes the cavities are partially occupied with it in liquid 
form, and the two fluids, as originally observed by Brewster, may be seen 
in the same cavity unmingled, the carbon-dioxide remaining as a freely 
moving globule within the carbonated water. Cubic crystals of chloride 
of sodium may be occasionally 
observed in the fluid, which must (^ ^n\ 

in such cases be a satui^ated _ /^ v^ 

solution of this salt (Fig. 11, 
lowest figure in Colunm A). 
Usually each cavity contains a 
small globule or bubble, some- 
times stationary, sometimes mov- 
able from one side or end of the 
cavity to the other, as the 
specimen is turned. With a 

high magmfying power, the Fjg.n.-Cavitiesln crystals, highly magnified; a. Liquid 
minuter bubbles may be observed inclusions ; b, Olass inclusions -. c, cavities showing 

to be in motion, sometimes slowly ^^e devitrification of the original glass by the appear- 

pulaating from side to side, or r/^^I^Lti'p^'uctt 1™^ """'' "^^ ' 
rapidly vibrating like a living 

organism. The cause of this trepidation, which resembles the so-called 
"Brownian movements," has been plausibly explained by the incessant 
interchange of the mole6ules from the liquid to the vaporous condition 
along the surface where vapours and liquid meet — an interchange which, 
though not visible on the large bubbles, makes itself apparent in the 
minute examples, of which the dimensions are comparable to those of the 
intermolecular spaces.^ The bubble may be made to disappear by the 
application of heat. 

With regard to the origin of the bubble, Sorby pointed out that it can 

1 ChArfoonelle and Thirion, Rev. Quest, Scientif. vii. (1880) 43. On the critical point of 
water, &c., in these cayities see Hartley, Joum. Chein, Soc. ser. 3, vol. ii. p. 241. Pop. Scu 
Rev. new aer. L p. 119. 






112 GEOGNOSY Booxn 

be imitated in artificial crystals, in which he explained its existence by 
diminution of volume of the liquid owing to a lowering of temperature 
after its enclosure. By a series of experiments he ascertained the rate of 
expansion of water and saline solutions up to a temperature of 200° G 
(392' Fahr.), and calculated from them the temperature at which the 
liquid in crystals would entirely fill its enclosing cavities. Thus, in Uie 
nepheline of the ejected blocks of Monte Somma, he found that the 
relative size of the vacuities was about *28 of the fluid, and assuming the 
pressure under which the crystals were formed to have been not much 
greater than sufficient to counteract the elastic force of the vapour, he 
concluded that the nepheline may have been formed at a tempera- 
ture of about 340° C. (644'' Fahr.), or a very dull red heat> only just 
visible in the dark. He estimated also from the fluid cavities in the 
quartz of granite that this rock has probably consolidated at somewhat 
similar temperatures, under a pressure sometimes equal to that of 76,000 
feet of rock.^ Zirkel, however, has pointed out that even in contiguooi 
cavities, where there is no evidence of leakage through fine fissures, the 
relative size of the vacuole varies within very wide limits, and in such a 
manner as to indicate no relation whatever to tlie dimensions of the 
enclosing ca\dties. Had the vacuole been due merely to the contractioii 
of the liquid on cooling, it ought to have always been proportionate to 
the size of the cavity.- 

MM. De la Vallee Poussin and Renard, attiicking the question from 
another side, measured the relative dimensions of the vesicle and of its 
enclosed water and cube of rock-salt, as contained in the quartziferoos 
diorite of Qucnast in Belgium. The temperature at which the ascertained 
volume of water in the cavity would dissolve its salt was found by calcub- 
tion to be 307'' C. (520' Fahr.) But as the law of the solubility of 
common salt has not l>een experimentally determined for high tempera- 
tures, this figure can only be accepted provisionally, though other 
considerations go to indicate that it is probably not far from the truth. 
Assuming then that this wa5 the temperature at which the vesicle was 
formed, these authors proceed to detennine the pressure necessary to 
prevent the complete vaporization of the water at that temperature, and 
obtain, lus the result, a pressure of 87 atmospheres, equal to 84 tons per 
s([uare foot of surface.^ That many rocks were formed under great pressuro 
is well shown ])y the liquid carbon-dioxide in the pores of their crystals. 

Although, in almost all cases, the liquid inclusions are to be referred 
to the conditions under which the mineral crystallized out of the original 
magma, they may be exceptionally developed long subsequently, either 
in one of the original minerals during decomposition, or in a mineral erf 
secondary origin, such as quartz of subsequent introduction.* 

1 Sorby, Q. J. Geol. ^>c. xiv. pp. 480, 493. 2 i^ik. Beschaff.' p. 46. 

^ * Memoire sur les Roches dites Plutoniennes de la Belgique, ' De la Vallee Pouasin and 
A. Reuard, Aciuf. Roy. Belg. 1876, p. 41. See also Ward, Q. J. Geol. Soc. xxxL p. 668, 
who believed that the granites of Cuniberlaud consolidated at a maximum depth of 22,000 
to 30,000 feet. 

* See Whitman Cross on the development of liquid inclusions in plagioclase dnring tbe 



PART II g V MICROSCOPIC CHARACTERS OF ROCKS 113 

Liquid inclusions may ]ye dispersed at random through a crystal, or 
as in the quartz of granite, gathered in intersecting planes (which look 
like fine fissures and which may sometimes have become real fissures, 
owing to the line of weakness caused by the crowding of the cavities), or 
disposed regularly in reference to the contour of the crystal. In the last 
case they are sometimes confined to the centre, sometimes arranged in 
zones along the lines of growth of the crystal.^ They are specially con- 
spicuous in the quartz of granite and other massive rocks, as well as of 
gneiss and mica -schist ; also in felspars, topaz, beryl, augite, nepheline, 
olivine, leucite and other minerals. 

y. Inclusions of glass or of some lithoid substance. — In many 
rocks which have consolidated from fusion, the component crystals contain 
globules or irregularly shaped enclosures of a vitreous nature (Fig. 11, 
Column B). These enclosures are analogous to the fluid-inclusions just 
described. They are portions of the original glassy magma out of which 
the minerals of the rock crystallized, as portions of the mother-liquor are 
enclosed in artificially formed crystals of common salt. That magma is 
in reality a liquid at high temperatures, though at ordinary temperatures 
it becomes a solid. At first, these glass-vesicles may be confounded with 
the true liquid-cavities, which in some respects they closely resemble. 
But they may be distinguished by the immobility of their bubbles, of 
which several are sometimes present in the same cavity ; by the absence 
of any diminution of the bubbles when heat is applied ; by the elongated 
shape of many of the bubbles ; by the occasional extrusion of a bubble 
almost beyond the walls of the vesicle ; by the usual pale greenish or 
brownish tint of the substance filling the vesicle, and its identity with 
that forming the surrounding base or ground-mass in which the crystals 
are imbedded ; and by the complete passivity of the substance in polarized 
light (see p. 94). 

Glass inclusions occur abundantly in some minerals, aggregated in the 
centre of a crystal or ranged along its zones of growth with singular 
regularity. They appear in felspars, quartz, leucite, and other crystalline 
ingredients of volcanic rocks, and of course prove that in such positions 
these minerals, even the refractory quartz, have undoubtedly crystallized 
out of molten solutions. 

In inclusions of a truly vitreous nature, traces of devitrification may 
not infrequently be seen. In particular, microscopic crystallites (p. 115) 
make their ap|)earance, like those in the ground -mass of the rock. 
Sometimes the inclusions, like the general ground-mass, have an entirely 
stony character (Fig. 11, C). This may be well observed in those which 
have not been entirely separated from the surrounding ground-mass, but 
are connected with it by a narrow neck at the periphery of the enclosing 
crystal. In some granites and in elvans, the quartz by irregular contrac- 
tion, while still in a plastic state, appears to have drawn into its substance 

decomposition of the gneiss of Brittany. Tschermak's Min. Mittheil. 1880. p. 369 ; also 
G. F. Becker, * Geology of Comatock Lode.' V. S. Geol. Sun\ 1882, p. 371. 

* The way in which vesicles, enclosed crystals, &c.. are grouped along the zones of 
growth of crystals is illustrated in Fig. 12. 

I 




of a fncluml lUid coirodeil A 
a dykd, Crawfortjnhii, L»D»rki 
lowing Llnei of growth with wi 



114 GEOGNOSY BOOi n 

portionB of the surrounding already lithoid base ; ' but this appeamnce 
may sometimes be due to irregular corrosion of the crystals by the magmiL* 
6. Crystals and crystalline bodies. — Man; component 
minerals of rocks contain other minerals (Fig. 13). These occur some- 
times as perfect crystals, more 
usually as what are termed micro- 
lit«s (p. 115). Like the ^bm- 
inclusions, they tend to range 
themselves in Unes along t£e 
successive zones of growtJi in 
the enclosing mineraL Micro- 
lites are of frequent occurrence 
in leucite, garnet, augite, horn- 
blende, calcite,fluorite,&c From 
the fact that microlites of the 
easily fusible augite are, in the 
Vesuvian lavaa, enclosed wit^ 
the extremely refractory leucite, 
it was supposed that the relative 
order of fusibility is not always 
followed in the microlites and 

^ enveloping crystals. But this 

has been satisfactorily explained 
by Fouquii and Miche!-L*vy, who have shown experimentally that leudtfl^ 
when crystallizing from fusion, tends to catch up inclusions of the sur- 
rounding glass, which, should the glass be pyroxenic, may assume the 
form of augite.' 

(. Filaments, streaks, patches, discoloration s. — 
Besides the enclosures already enumerated, crystals likewise frequently 
enclose iiTegular portions of mineral matter, due to alteration of the 
original substance of the minerals or rocks. Thus tufta and vermicular 
aggregates of certain green ferruginous silicates are of common occur- 
rence among the crystals and cavities of old pyroxenic volcanic rocks. 
Orthoclasc crystals are often mottled with patches of a granular nature, 
due to partial conversion of the mineral into kaolin. The magnetite, so 
frequently enclosed within minerals, is abundantly oxidized, and haa 
given rifle to brown and yellow patches and discol orations. Care must 
be taken not to confound these results of infiltrating water with the 
original characters of a rock. Practice will give the student confidence 
in distinguishing them, if he familiarises his eye with decomposition 
products by studying slices of weathered minerals and of the weathered 
parts of rocks. 

B. Glass.— Even to the unassist«d eye, many volcanic rocks consist 
obviously in whole or in great measure of glass.* This sul^tance in 

' J. A. Phillips, Q. J. Orol. Sac. xxxi. iJ. 388. 

- Pquhub and Michel-Wvy, 'Miii. Micrograph.' 

' 'Sj-nthiae dea Miniiraui,' 1882, p. 155. 

* See E. CoheD an glasny Bocks. iV«w> JaJirb. 1S80 (ii.), p. 23. 



iBTii § V MICROSCOPIC CHARACTERS OF ROCKS 116 

lass is usually black or dark green, but when examined in thin sections 
nder the microscope, it presents for the most part a pale brown tint, or 
\ nearly colourless. In its purest condition, it is quite structureless, 
bat is, it contains no crystals, crystallites, or other distinguishable 
idividualised bodies. But even in this state it may sometimes be 
bserved to be marked by clot-like patches or streaks of darker and 
ighter tint, arranged in lines or eddy-like curves, indicative of the flow 
f the original fluid mass. Rotated in the dark field of crossed Nicol- 
orisms, such a natural glass remains dark, as, unless where it has under- 
gone internal stresses, it is perfectly inert in polarized light Being thus 
wiropiCy it may readily be distinguished from any enclosed crystals which, 
cting on the light, are anisotropic (p. 94). Perfectly homogeneous 
tructnreless glass, without enclosures of any kind, occurs for the most 
«rt only in limited patches, even in the most thoroughly vitreous rocks. 
hriginally the structure of all glassy rocks, at the time of most complete 
ludon, may have been that of perfectly unindividualised glass. But as 
heee masses tended towards a solid form, devitrification of their glass 
et in. Many forms of incipient or imperfect crystallization, as well as 
lerfect crystals, were developed in the still fluid and moving mass, 
nd, together with crystals of earlier growth, were aiTanged in the 
lirection of motion. Devitrification has in frequent examples proceeded 

far that no trace remains of any actual glass.^ 

C. Crystallites and Microlites.^ — Under these names may be 
oehided minute inorganic bodies possessing a more or less definite form, 
»at generally without the geometrical characters of crystals. They occur 
Qoet commonly in rocks which have been formed from igneous fusion, 
lut are found also in others which have resulted from, or have been 
Itered by, aqueous solutions. They seem to be early or peculiar forms 

1 crystallization. They are abundantly developed in artificial slags, 
nd appear in many modern and ancient vitreous rocks, but the 
onditions imder which they are produced are not yet well understood.^ 

Crystallites are distinguished by remaining isotropic in polarized 
ight The simplest are extremely minute drop-like bodies or globuliteSy 
ometimes crowded confusedly through the glass, giving it a dull or 
omewhat granular character, while in other cases they are arranged in 
ines or groups. Gradations can be traced from spherical or spheroidal 
jlobulites into other forms more elliptical in shape, but still having a 
oonded outline and sometimes sharp ends (longulites). There does not 

^ Consult a paper on the microscopic character of devitritied glass and some analogous 
9ck-stmctares, by D. Herman and F. Rutley. Proc, Roy, Soc 1885, p. 87. 

' The word cryatalliU was first used by Sir James Hall to denote the lithoid substance 
bteined by him after fusing and then slowly cooling various ** whinstones " (diabases, &c.) 
ineo its leviTal in lithology it has been applied to the .minuter bodi&s above described. 
iMstadeDt should consult Vogelsang's ' Philosophie der Geologie, ' p. 139; ' Krystalliteu, ' 
ionii, 8vo, 1875 ; also his descriptions in Archives yeerlandaises, v. 1870, vi. 1871. 
orby, Brxi, Auoc, 1880. 

* They are well exhibited also in ordinary blow-pii)e beads. See Sorby, Brit. Assoc, 
880, or Otol, Mag. 1880, p. 468. They have been produced experimentally in the 
rtiildal rocks fused by Messrs. Fouque and Michel-L<'vy. 



ajjpcar to be any essential distinction, save in degiee of development^ 
between these forms and the long rod-like or needle-shaped bodies which 
have been termed Monilcs. Existing somedmes as mere simple needles 
or rods, these nioi'c elongated crystallites may be traced into more 
complex forms, cun-cd or coiled, at one time solitary, at another in 
groups. Ill most casus, crystallites are transparent and colourless, or 
slightly tinted, but sometimes they are black and opatjue, from a coating 
of ferruginous oxide, or only appear so as an optical delusion from their 
position. Black, seemingly o|Mqiie, hair-like, twisted and cm-ved formi^ 
termed IricliiUn, occur abundantly in obsidian. 

Microlites are other incipient forms of crystallization which differ 
from crystallites in that they react on ]^)olarized light. They assume rod- 
like or needle-shaped forms sometimes occurring singly, sometimes in 
aggregates, and even occasionally grouped into skeleton-crystals. They 
can for the most jtart l>e identified as rudimentary forms of definite 
minerals such as augite, hornblende, felsjiar, olivine, and magnetite. 

Good illiutrations of the general charLtcter and grouping of crystallites 
and microlites are shown in some vitreous basalts. In Fig. 13, for 




— AuRltf Cr}»t«l aumiuiulni hy Crjii- 
Ullitrw «n.l MitrolitvK, fr.™ thr vitn.-"i» 
AioU'ilt.^ Kf Bskdalrnmlr, iiiajnilHi.<l SOU 




exaniple, the outer jmrtion of the field displays crowded globulites and 
longulites, as well as here and thei'e a few belonites and some curved and 
coiled trichites- Round the rude augite crystal, these various bodies have 
been drawn together out of the surrounding glass. Numerous rod-like 
microlites diverge from the crystal, and these are moi-e or less thickly 
crusted with the simpler and smaller forms.' In Fig. H, the remarkably 
1>eautiful stnicture of an Arran pitchstone is shown ; the glassy base 
Iwing crowded with minute microlites of horrd>lende which are grouped 
in a fine feathery or brush-like arrangement round tai>ering rods. In 
this case, also, we see that the glassy base has been clarified round the 
larger individuals by the altstraction of the crowded smaller microliter. 
6, Plnl» V. Vlg. S. J. J. H. Teall, Q. J. Oeot. 



PART 11 § V MICROSCOPIC CHARACTERS OF ROCKS 117 



By the progressive development of crystallites, microlites, or crystals 
during the cooling and consolidation of a molten rock, a glass loses its 
vitreous character and becomes lithoid ; in other words, undergoes 
devitrification. 

The characteristic amorphous or indefinitely granular and fibrous or 
scaly matter, constituting the microscopic base in which the definite 
crystals of felsites and porphyries are imbedded (pp. 160, 161), has 
been the subject of much discussion. Between crossed Nicol-prisms it 
sometimes behaves isotropically, like a glass, but in other cases allows a 
mottled glimmering light to pass through. It is now well understood to 
be a product of the devitrification of once glassy rocks wherein the 
crystallitic and microlitic forms can still be recognised or have been more 
or less effaced by subsequent alteration by infiltrating water. ^ 

Every gradation in the relative abundance of crystallites may be 
traced. In some obsidians and other vitreous rocks, portions of the 
glass can be obtained with comparatively few of them ; but in the same 
rocks we may not infrequently observe adjacent parts where they have 
been so largely developed as to usurp the place of the original glass, and 
give the rock in consequence a lithoid aspect (Fig. 11, C and pp. 160-3). 

D. Dktritus. — Many rocks are composed of the detritus of pre-exist- 
ing materials. In the great majority of cases this can be readily detected, 
even with the naked eye. But where the texture of such detrital or 
fragmental (clastic) rocks becomes exceedingly fine, their true nature may 
require elucidation with the microscope (Figs. 21, 22). An obvious dis- 
tinction can be drawn between a mass of compact detritus and a crystalline 
or vitreous rock. The detrital materials are found to consist of various and 
irregularly shaped grains, with more or less of an amorphous and generally 
granular paste. In some cases, the grains are broken and angular, in 
others they are rounded or waterworn (pp. 128, 129). They may consist 
of minerals (quartz, chert, felspars, micii, t^'c), or of rocks (slate, limestone, 
basalt, &c.), or of the remains of plants or animals (spores of lycopods, 
fragments of shells, crinoids, <fcc.) It is evident therefore that though 
some of them may be crystalline, the rock of which they now fonn part 
is a non-crystalline compound. Water, with carbonate of lime or other 
mineral matter in solution, permeating a detrital rock, has sometimes 
allowed its dissolved materials to crystallize among the interstices of the 
detritus, thus producing a more or less distinctly crystalline structure. 
But the fundamentally secondary or derivative nature of the mass is not 
always thereby effaced. 

2. Microscopic Structures of Bocks, 

We have next to consider the manner in which the foregoing 
microscopic elements are associated in rocks. This inquiry brings before 
us the minute structure or texture of rocks, and throws great light upon 
their origin and history.^ 

^ See Zirkel, * Mik. Beschaff.' p. 280. Roseubusch, vol. ii. p. 60. 

' The first broad classification of the microscopic structure of rocks was that proposed 
by Zirkel, which, with slight moditicatiou, is here adopted. 'Mik. Beschafi*.* \k 265. 



118 QEOGNOSY bookh 

Four types of rock-atnicture arc revealed by the microBCOpo. A, 
holocrystalliiie ; B, henii-cryatalline ; C, glaeay ; D, clastic. 

A. HoLOCRYSTALLiNE, conBistiiig entirely of crystala or crystalline 
indinduals, whether visible to the naked eye, or I'equiring the aid of ■ 
microscope, imbedded in each other without any intervening amorphoiu 
substance. Rocks of this tj-jtc are exemplified by granite (Figs. 15 and 29) 
and by other igneous rocks. But they occur also among the cryBtalline 
limestones and schists, as in stittuarv marble, which consist* entirely of 
crystalline granules of calcite (Fig. 28), 

According to the classification proj)osed by Prof. Rosenbusch Um 
holocrystalline structure is idwmm-phic or panidiomorphic when each ot 
the component crystals has assumed its own crystal lographic form, and 
iillntriomtyrphic when it has its outlines determined by those of its neighbounL 
When interspaces have been left between the crystals or crystalline grains 
the Btnictiire is minrolilir or saccharoid. 

The holocrystalline eruptive rocks (p. 164) are typically represented 
by granite, hence the term <franitm<l has been adopted to express their 




Bw trirliDli- Fclainr : tho bnader mi 
roniii, illghUr ahided in die dnvtng, m 
AUKltr : the lilsck apecka an Htgnatlte; 
the nwlle-Bhaped rorma >n AjuXMk. <Bm 

microscopic stnicture. Varieties of this structure are designated 
according to the relations of the component minerals. ^V'here no one 
mineral greatly preponderates, but where they are all confusedly and 
tolerably equally distributed in individuals readily observable by the 
naked eye, as onlinary granite, the structure is granUic (see gnmviar, 
p. 99). \\Tiere a similar stnicture is so fine that it can only be re- 
cognised with the microsco]>e, it has been called micro^-a-nUie or tm-Uic. 
Where the minerals arc gronjicd in small, isolated, grain-like individuals, 
' Bualt^9i|«iQe,' p. 88. 8™ nlso Roscnbusoh's sngRBstive [Kiper already cited, A'etiet JaJiti, 



PAST n g T MICROSCOPIC CHARACTERS OF ROCKS 119 

each having its own independent crjBtalline structure, so that under the 
microscope in polarized light, the rock presents the appearance of a 
brilliant mosaic, the structure has been named granulilic or micro^anuHiic 
{ panidicmorphic ffranvlar OT pi»yhyric oi RosenhuBch). Where the quartz 
and felspar of a granitic rock have crystallized together, one within the 
other, the atmcture is pe^matUic (Fig. 31) where visihle to the naked eye, 
and micropegmalitie {graaophyrie of Rosenbusch) where the help of a 
microscope is needed (Fig. 5).^ 

R Hemi-CRTSTALLINE.' — This division probably comprehends the 
majority of the massive eruptive or igneous rocks. It is distinguished 
by the occurrence of what appears to the naked eye as a compact or 
finely granular ground-mass, through which more or less recognisable 
crystals are scattered. Examined with the microscope, this ground-mass 
is found to present considerable diversity (Figs. 16, 18, 32). It may be 
(1) wholly a glass, as in some basalts, trachytes, and other volcanic 
products; (2) partly devitrified through separation of peculiar little 
granules and needles (crystallites and microlitcs) which appear in a vitreous 
base ; (3) still further devitrified, until it becomes an aggregation of such 
little granulea, needles, and hairs, between which little or no glass-base 
appears (microerystallitic) ; or (4) " microfelsitic " (petrosillccous), closely 
related to the two previous groups, and consisting of a nearly structureless 
— 'T". marked usually with indefinite or half-effaced granules and filaments, 
bat behaving like a singly-refracting, amorphous body (p. 1 1 5). 

In rocks belonging to this type, a spherulitic structure has sometimes 




been produced by the appearance of globular bodies composed of a 

1 Foaqa^ and Mlcbel-L^vy, ' Min. Micn^rajili.' The niicropegniBtite of Mii-liel-L^vy 
ii the Mmc u the structure sabstqucatly named grBoapliyre by RoBeabnsch. Michel 
Uvy, 'Roches Ernptlvea.' p. 19. 

* Pot this rtructure the term " miierl " haa been proposed, a.« being a mixture of tlie 
cijit^Iiue and amorphona (glaaay) strnctures. It has becu deai^ated by Fouiiue and 
Hichd-UTjr " tr«chytoid, " u being typically developed among the trachytes {potlra p. 
IM). It U called "hypocryntalline " by Rosenbusch. 



120 GEOnXOsy book n 

crystal! i HO internally radiating substance, sometimes with concentric 
i>he)ls of amorphous material. In many caiieA, spheiailites nre only 
recognisable with the microscojjc, when they each present a bbick eros* 
between crossed Xicol-prisma, and thereby characteristically reveal the 
micnmpkeriiliHc stiiicture (Figs. 7 and 17),' 

The term ophitic is applied to a rftnicture in which one mineral aft«r 
ciystallizing has been enclost^ within another during the consoiitlation of 
an ifpieouK ruck (Fig. Ir*). It is abundant in many dolerites and diabases 
where some bisilicate siich as angite senes as a matrix in which the 
fels|)ars and other crystals are enclosed. The name ie derived from tha 
so-called " ophites " of the PyrenoeB.- 

C. Gla-S-sy. — Composed of » volcanic glass such as has already been 
described. It seldom happens, however, that rocki: which seem to the eye 
to be tolerably homogeneous glass do not contain abundant cr^'stallite* 
and minute crj'stals. Hence truly vitreous rocks tenil to graduate into 
the second or hemi-cni'stalline tyjx'. This gradation and the abundant 
ti-aces of a deritrified base or magma lictween the crystals of a vast 
nnniber of eniptive rocks, lead to the belief that the glassy type was the 
original condition of most if not all of these rocks. Enipted as molten 
masses, their mobility wonlil depend u]H>n the fluidity of the glass. Yet 
even while still deep within the eai'th's crust, some of their constituent 
minerals (felspars, leucite, magnetite, Ac.) were often already crystallized, 
and suflered fracture and corrosion by su1tse(|Upnt action of the enclosing 
magma. This is well shown by what is termed the Jtoitsirueliire or 




stallites are rangeil in current-like lines, 

t ion of these lines. Where a large older 

ter individuals is found to siveep round 



J. Kiiliii. XalscJi. IhaUch. l/fi. 



PART u § V MICROHGOPIC CHARACTERS OF ROCKS 121 

it and to reunite on the further side, or to be diverted in an eddy-like 
course (Fig. 19). So thoroughly is this arrangement characteristic of the 
motion of a somewhat viscid liquid, that there cannot be any doubt that 
such was the condition of these masses before their consolidation. The 
flow-structure may be detected in many eruptive rocks, from thoroughly 
vitreous compounds like obsidian, on the one hand, to completely crystal- 
line masses like some dolerites, on the other. It occurs not- only in what 
are usually regarded as volcanic rocks, but also in plutonic or deep-seated 
masses which, there is reason to believe, consolidated beneath the surface, 
as for instance in the Bode vein of the Harz, among quartz-porphyries 
associated with granites in Aberdeenshire, and in felsite dykes and bosses 
in the Shetlands, Skye, central Scotland, and County Waterford. The 
structure, therefore, cannot be regarded as certainly indicating that the 
rock in which it is found ever flowed out at the suriface as lava. 

Some glassy rocks, in cooling and consolidating, have had spherulites 
developed in them (Fig. 17) ; also by contraction the system of reticulated 
and spiral cracks known as perlitic structure (p. 101 and Figs. 9 and 20). 

The final stiffening of a vitreous mass into solid stone has resulted 
(Ist) from mere solidification of the glass : this is well seen at the edge 
of dykes and intrusive sheets of different basalt-rocks, where the igneous 
mass, haling been suddenly congealed along its line of contact with the 
surrounding rocks, remains there in the condition of glass, though only 
an inch further inward from the edge the vitreous magma has dis- 
appeared, as represented in Fig. 287; (2nd) from the devitrification of 
the glass by the abundant development of microfelsitic granules and 
filaments, as in quartz-porphyry, or of crystallites, microlites and crysUils, 
as in such glassy rocks as obsidian and tachylite ; or (3rd) from the 
complete crystallization of the whole of the onginal glassy base, as may 
be obeerved in some dolerites. 

D. Clastic. — Composed of detrital materials, such as have been already 
described (p. 103 and Fig. 2 1 ). Where these materials consist of grains of 
quartz-sand, they withstand almost any subsequent change, and hence 
can be recognised even among a highly metamorphosed series of rocks. 
Quartzite from such a series can sometimes be scarcely distinguished under 
the microscope from unaltered quartzose sandstone. Where the detritus 
has resulted from the destruction of aluminous or magnesian silicates, 
it is more susceptible of alteration. Hence it can be traced in regions 
of local metamorphism, becoming more and more crystalline, until the 
rocks formed of or containing it pass into true crystalline schists. 

Detritus derived from the comminution or decay of organic remains 
presents very different and characteristic structures ^ (Fig. 22). 
Sometimes it is of a siliceous nature, as where it has been derived from 
diatoms and radiolarians. But most of the organically-derived detritiil 
rocks are calcareous, formed from the remains of foraminifera, corals, 
echinoderms, i)olyzoa, cirripedes, annelides, mollusks, Crustacea and 

* The student who would further investigate tliis subject, will tin«l a suggestive and 
laininoufl essay upon it by Mr. Sorby in liis Presidential Address to the Geological Society. 
Qtuirt. Journ, Otol, Sue. 1879. 



other iiivertel)rate8, «-iih <M:casioiial traces of fishes or even of higher 
vertebrates. Distinct differences of microscopic structure can be detected 




kii'tim', or Orguile OrlglB- 
Htmctun of Cluilk (Sorb]). Migniflrd IM 
mttm. !4W1>.132.) Diainet«ni. {Sf<- |i. 140l) 

in the hanl iwrts of some of the livn'ng representatives of these fornu, 
and similar diKerences have been detected in beds of limestone of all age^ 
Mr. Sorby, in the i>a))er cited below, has shown how characteristic and 
persistent are some of these distinctions, and how they may be mode to 
indicate the origin of the rock in which they occur. There is ui 
important difference between the two forms in which carbonate of litiie 
in made use of by invertebrate animals ; aragonite being much lesi 
durable than calcito (pp. 78, 139). Hence while shells of gasteropoda, 
many lamellibninchB, corals and other organisms, formed largely w 
wholly of aragonite, crumble down into mere amorphous mud, pass into 
crystalline calcite, or disappear, the fragments of those consisting of 
calcito may remain (luite recognisable. 

It is evident, therefore, that the absence of all trace of organic 
structure in a limestone need not invalidate an inference from other 
evidence that the rock has been formed from the remains of oi^nienu. 
The calcareous organic debris of a sea-bottom may he disintegrated, and 
reduced to amorphous detritus, by the mechanical action of wave* and 
currents, by the solvent chemical action of the water, liy the decay of 
the binding matei-ial, such as the organic matter of shells, or by being 
swallowed and digested by other animals {pnska, p. 138).' 

Moreover, in clastic calcareous rocks, owing to their liability to alter- 
ation by infiltrating water, there is a tendency to aujuire an internal 
crystalline texture (p. 366). At the time of foi-mation, little empty spocea 
lie between the component granules and frai^ents, and according to Mr. 
Sorby, these interspaces may amount to alrout a quarter of the whole 
mass of the rock. They have very commonly bceti tilled up by calcite 
introtluced in solution. This infiltrated calcite itcquircs a crystalline 
' Sorby, Preaiiienlinl Aitdrem, (i. J. Hfol. .■4ic 187i>. 0. Rom. Ablian^. Amd. Berlin, 
1858 ; Oiimbel, XeilKh. DeuHch. f/fni. Gmellach. 1884, {i. 386. Cornish moA Kendall, StU, 
Mag. 1888, p. C6. 



PART n § vi CLASSIFICATION OF ROCKS 123 

structure, like that of ordinary mineral-veins. But the original com- 
ponent organic granules also themselves become crystalline, and, save 
in so far as their external contour may reveal their original organic 
source, they cannot be distinguished from mere mineral-grains. In this 
way, a cycle of geological change is completed. The calcium-carbonate 
originally dissolved out of rocks by infiltrating water, and carried into 
the sea, is secreted from the oceanic waters by corals, foraminifera, 
echinoderms, moUusks and other invertebrates. The remains of these 
cretttares collected on the sea-bottom slowly accumulate into beds of 
detritus, which in after times are upheaved into land. Water once 
more percolating through the calcareous mass, gradually imparts to it a 
crystalline structure, and eventually all trace of organic forms may be 
effaced. But at the same time, the rock, once exposed to meteoric 
influences, is attacked by carbonated water, its molecules are carried in 
solution into the sea, where they will again be built up into the frame- 
work of marine organisms. 

K Alteration of Rocks by Meteoric Water. — An impoitant 
revelation of the microscope is the extent to which rocks suffer from the 
influence of infiltrating water. The nature of some of these changes is 
described in subsequent pages. (Book III. Part II. Sect. ii. § 2.) It may 
be sufficient to note here a few of the more obvious proofs of alteration. 
Threads and kernels of calcite running through an eruptive rock, such 
as diabase, dolerite, or andesite, are a good index of internal decomposi- 
tion. They usually point to the decay of some lime-bearing mineral in 
the rock. Some other minerals are likewise frequent signs of alteration, 
such as serpentine (often resulting from the alteration of olivine (Figs. 
33, 34), chlorite, epidote, limonite, chalcedony, &c. In many cases, 
however, the decomposition products are so indefinite in form and so 
minute in quantity, as not to permit of their being satisfactorily referred 
to any known species of mineral. For these indeterminate, but 
frequently abundant substances, the following short names were 
proposed by Vogelsang to save periphrasis, until the true nature of the 
substance is ascertained. Viridite — green transparent or translucent 
patches, often in scaly or fibrous aggregations, of common occurrence in 
more or less decomposed rocks containing hornblende, augite, or olivine : 
{Hx>bably in many cases serpentine, in others chlorite or delessite. Ferrite 
— yellowish, reddish, or brownish amorphous substances, probably consist- 
ing of peroxide of iron, either hydrous or anhydrous, but not certainly 
referable to any mineral, though sometimes pseudomorphous after 
ferruginous minerals. Opaciie — black, opaque grains and scales of 
amorphous earthy matter, which may in different cases be magnetite, 
or some other metallic oxide, earthy silicates, graphite, <fcc.^ 

§ vi. Classification of Bocks. 

It is evident that Lithology may be approached from two very 
different sides. We may, on the one hand, regard rocks chiefly as so 

* VogelsaDg, Z. Deutach. OeoL Oes. xxiv. (1872), p. ri29. Zirkol, Oeol. ExpL iOt?i 
PetraiMf toL tL p. 12. 



1 2 4 GEOGXOS y BOOK II 



many masses of mineral matter, presenting great 'vanety of chemical 
comiK)sition and marvellous diversity of microscopic structure. Or, on 
the other hand, passing from the details of their chemical and mineral- 
ogical characters, we may look at them rather as the records of ancient 
t<>rrestrial changes. In the former aspect, they present for consideration 
problems of the highest interest in inorganic chemistry and mineralogy ; 
in the latter view, they invite attention to the gi'cat geological revolu- 
tions through which the planet htu* jmssod. It is evident, therefore, 
that two distinct systems of classification may l)e followed, the one 
iKised on chemical and mineralogical, the other on geological con- 
siderations. 

P'rom a chemical point of view, rocks may be gi'ouped according to 
their composition ; as Oxides, exemplified by foiTiiations of quartz, 
luematite, or magnetite ; Carbonates, including the limestones and 
clay-ironstones ; Silicates, embracing the vast majority of rocks, whether 
eomixosed of a single mineral, or of more than one ; Phosphates, such ae 
guano and the older bone-beds and coprolitic dej)osits. A classification 
of this kind, however, piys no regard to the mode of origin or conditions 
of occui-rence of the rocks, and is not well suite<l for the purposes of the 
geologist.^ 

P^rom the mineralogical side, rocks may be classified with reference 
to their prevailing minei'al constituent. Thus such subdiAisions as 
Calcareous rocks, Quartzose rocks, Orthoclase rocks, Plagioclase rocks, 
Pyroxenic rocks, Hornblendic rocks, &c., may be adopted ; but these 
terms are hardly less objectionable to the geologist, and are in fact 
suited rather for the arrangement of hand-specimens in a museum, than 
for the investigation of rocks in situ. 

From the standpoint of geological inquiry, rocks have been classified 
according to their mode of origin. In one system they arc arranged 
under three great divisions : 1st, If/nwus, embracing all which have been 
erui>ted from the heated interior of the earth ; 2nd, - Iqueous or SaHment- 
anf, including all which have been laid down as mechanical or chemical 
deposits from water or air, and all which have resulted from the growth 
and decay of plants or animals : 3rd, Metn ntorjihic, those which have 
undei'gone subsequent change within the crust of the eiuth, whereby 
their original character has l>een so modified as to be sometimes quite 
indeterminable. Another geological arrangement is l)ased upon the 
geneial structure of the rocks, and consists of two divisions, 1st, 
Sfratifu'fl, embiucing all the arpieous and sedimentary, with part of the 
less altered metumorphic rocks : 2nd, l/nsfrafijiid, nearly conterminous 
with the term igneous, since it includes all the erui)tivo rocks. Further 
suMivisions of this series have been proi>08cd according to differences of 
structure or texture, as jMtrphf/ritiCy granitic^ Szc. 'i'hese geological sulv 
divisions, however, ignore the chemic^d and mineralogical characters of 
the locks, and are based on deductions which may not always be sound. 
Thus, rocks may be included in the igneous series, which further research 

' The eruptive rook« are siisceptilile of a coiivi«iiieiit, though not strictly accurate, 
cheiiiicul (.'lassificution into acidj luterim'tliaft', an«l basic (sec \k ir)0). 



PART II g vi CLASSIFICATION OF ROCKS 125 



may show not to be of igneous origin ; others may be classed as meta- 
morphic, regarding the true origin of which there may be considerable 
uncertainty. 

A further system of classification, based upon relative age, has been 
applied to the arrangement of the eruptive rocks, those masses which 
were erupted prior to Secondary time being classed as "older," and 
those of Tertiary and later date as " younger." This system has been 
elaborated in great detail by Michel-L^vj', who maintains that the same 
types have been reproduced nearly in the same order in the two series, 
Uiough basic rocks, often with vitreous characters, rather predominate in 
the later. ^ It must, indeed, be admitted that certain broad distinctions 
between the older and the later eruptive rocks have been well ascertained, 
and appear to hold generally over the world. Among these distinctions 
may be mentioned as more characteristic of the Palaeozoic rocks the presence 
of microcline, tiu*bid orthoclase in Carlsbad twins, muscovite, enstatite, 
bronzite, diallage, tourmaline, anatase, rutile, cordierite, and in the 
younger rocks the presence of sanidine, tridymite, leucite, nosean, hauyne, 
and zeolites. Even where the same mineral occurs in both the older and 
newer series, it often presents a somewhat different aspect in each, as in 
the case of the plagioclase and augite, which in the younger series are 
distinguished by the occurrence in them of vitreous and gaseous in- 
clusions which are rare or absent in those of the older series.^ Throughout 
the younger eruptive rocks, the vitreous condition is much more frequent 
and perfectly developed than in the older group, where, on the other 
hand, the granitic structure is characteristically displayed. Still, to these 
rules so many exceptions occur that it may be doubted whether enough 
of positively ascertained data have been collected regarding the relative 
ages of eruptive rocks to warrant the adoption of any classifi cation upon 
a chronological basis. There can be no doubt that, making due allowance 
for the alterations arising from permeation by meteoric water, there is no 
essential difference between some types of volcanic rock in Palaeozoic and 
in recent times. The Carboniferous basalts and trachvtes of Scotland, for 
example, present the closest resemblance to those of Tertiary age.^ 

Though no classification which can at present be proposed is wholly 
satisfactory, one which shall do least violence, at once to geological and 
mineralogical relationships, is to be preferred. The arrangement which has 
met with the most general acceptance is threefold. 1st, Sedimentary 
Rocks, including first the rocks which have resulted from the accumulation 

* See on this subject, J. D. Dana, Awer. J. «Slct. xvi. 1878, p. 336. Michel-Levy, 
BM, Soc, OM, Frafice, 8rd ser. iii. (1874), p. 199; vi. p. 173. Ann. ihs Mines, viii. 
(1875) * Roches Eruptives,* 1889. Fouqueand Michel-Levy, 'Miueralogie Microgr.' p. ir»0. 
Rosenbusch, * Mik. Physiog.' ii. Reyer, 'Physikder Eruptionen,' 1877, part iii. opposes 
the mdoption of relative age as a basis of classification. On the classification of compound 
silicated rocks, see Vogelsang, Z. Deutsch. Oeol. Oes, xxiv. p. 507 : and for an incisive 
criticisni of a too merely mineralogical classification, Lossen, oj). cit. xxiv. p. 782. Consult 
also O. Lang, * Ober die Individaalitiit der Gesteine ' in Tsch^rmaJca Min. Mittheil. vol. xi. 
part 6 (1890), p. 467. 

* See J. Murray and A. Renard, Proc Hoy. Sot-. Edin. xi. p. 669. 
» See Xature, iii. (1871), p. 303. 



126 frKOirXOSY BOOKU 



of detritus, either inorganic or organic, under water or on land, and 
secondly those which have been deposited irom aqueous solution. The 
former are mechanical, the latter chemical accumulations ; but they have 
oiten been deposited together. Certain rocks of mechanical origin, such 
as detrital limestones, mav bv subseciuent alteration be converted into 
materials that cannot l)e distinguished from others of true chemical 
origin. Hence the whole series is intimately linked together. 2nd, 
Massive, Eruptive, or Intrusive Kocks, embiucing all those which 
have solidified from fusion within the earth's crust, or have been erupted 
as lava to the surface. 3rd, Schistose Rocks, and their accompani- 
ments, including the so-called Metamorphic rocks which have reached 
their pi-esent condition as a consequence of the alteration sometimes of 
sedimentiiry, sometimes of igneous rocks. This group graduates into the 
two others, but it contains some distinctive masses, the origin of which 
is still involved in doul)t. 

It must be kept in view that in this projwsed system of classificatioD, 
and in the following detailed description of rocks, many questions 
regarding the origin and decom])osition of these mineral masses must 
necessai-iiy be alluded to. The student, however, will find these ques- 
tions discussed in later pages, and will prol)ably recognise a distinct 
advantiige in this unavoidable ])reliminary reference to them in connec- 
tion with the rocks by which they are suggested. 

§ vii.— A Description of the more Important Rocks of the Earth's Cruatb 

Full detiiils regarding the comix>sition, microscopic structure, and 
other characters of rocks must be sought in such general treatises and 
sjK'cial memoirs as those already cited (pp. 89, 96, 108). The purposes 
of the present text-lx>ok will be served by a succinct account of the more 
common or imi)ortiUit rocks which enter into the composition of the 
criLst of the earth. 

I. Sedimentary. 

A. Fi:a«;mkntal (Cla-stic^ 

This great series eml)races all rocks of a secondary or derivative 
origin ; in other words, all formed of fragmentary materials which have 
previously existed on or l^eneath the surface of the eaith in another 
form, and the accumulation and consolidation of which gives rise to new 
compoiuids. Some of these materials have been produced by the 
mechanical action of wind, as in the sand-hills of sea-coasts and inland 
deserts (-L-Eolian rocks) ; othei^s by the operation of moving water, as the 
gravel, sand and mud of shores and river-beds (Aqueous sedimentary 
rocks) ; others ])y the accumulation of the entire or fragmentary remains 
of once living plant*; and animals (Organically-formed rocks) ; while yet 
another series has arisen from the gathering together of the loose debris 
thrown out by volcanoes (Volcanic tuffs). It is evident that in dealing 



PARTii§vii FRAGMENT AL ROCKS 127 

with these various detrital formations, the degree of consolidation is of 
secondary importance. The soft sand and mud of a modern lake-bottom 
differ in no essential respect from ancient lacustrine strata, and may tell 
their geological story equally well. No line is to be drawn between 
what is popularly termed rock and the loose, as yet uncompacted, debris 
out of which solid rocks may eventually be formed. Hence in the 
following arrangement, the modem and the ancient, being one in structure 
and mode of formation, are classed together. 

It will be observed that^ in several directions, we are led by the frag- 
mental rocks to crystalline stratified deposits, some of which have been 
deposited from chemical solution, while others have resulted from the 
gradual conversion of a detrital into a crystalline structure. Both series 
of deposits are accumulated simultaneously and are often interstratified. 
Calcareous rocks formed of organic remains (p. 138) exhibit very clearly 
this gradual internal change, which more or less effaces their detrital 
origin, and gives them such a crystalline character as to entitle them 
to be ranked among the crystalline limestones. 

1. Gravel and Sand Bocks (Psammites). 

• 

As the deposits included in this subdivision are produced by the disintegration and 
remoTal of rocks by the action of the atmosphere, rain, rivers, frost, the sea, and other 
saperficial agencies, they are mere mechanical accumulations, and necessarily vary 
indefinitely in composition, according to the nature of the sources from which they are 
derired. As a rule, they consist of the detritus of siliceous rocks, these being among 
the most durable materials. Quartz, in }»articular, enters largely into the composition 
of aendy and gravelly detritus. Fragmentary materials tend to group themselves 
according to their size and relative density. Hence they are apt to occur in layers, and 
to show the characteristic stratified arrangement of sedimentary rocks. They may 
enclose the remains of any plants or animals entombed on the same sea-floor, river-bed, 
or lake-bottom. 

In the nu^ority of these rocks, their general mineral composition is obvious to the 
naked eye. But the application of the microsco])e to their investigation has thrown 
considerable light U{K>n their com{)osition, formation, and subsequent mutations. Their 
component materials are thus ascertained to be divisible into — 1st, derived fragments, of 
which the most abundant are qu^lz, after which come felspar, mica, iron-qres, zircon, 
mtile, apatite, tourmaline, garget, sphene, augite, h ornble nde, fragments of various 
rocks, and clastic dust ; 2nd, constituents which have been deposited between the 
paiticlee, and which in many cases serve as the cementing material of the rock. Among 
the more important of these are silicic acid in the form of quartz, chalcedony and oj>a) ; 
carbonates of lime, iron or magnesia ; haematite, limonite ; ])yrite and glauconite.^ 

dUMMbria, Moraine Stnif— ^angular rul)bish disengaged by frost and ordinary 
atmospheric waste from cliffs, crags, and steep slopes. It slides down the declivities of 
hilly regions, and accumulates at their base, until washed away by rain or by brooks. 
It forms talns-sloi>es of as much as 40*", though for short distances, if the blocks 
are laige, the general angle of slope may be much steeper. It naturally dei)ends for 
its composition upon the nature of the solid rocks from which it is derived. Where 
clifT-debriB iaUs upon and is borne along by glaciers it is called ''Moraine-stuff," which 



1 G. Klemm, ZeUsch, Deutsch. Oecl, Ges, xxxiv. (1882), p. 771. H. C. Sorby, Quart. 
Jtmrn, Otol. Soe, xxxvi. (1880). J. A. PhiUips, op. cit. xxxvii. (1881), p. 6. 



12H fiEfffwyosy BOOK It 



iiiav !»♦.• «I»-jni>it»^i m-rti- its SMiinr. or may \**r traiisiM>rte«l f«"»r many milcr^ ou the :$urfjic<: of 
til- i«.f p. A'2'-i . 

Perched Blocki. Erratic Blocki — lai-^- ma.ss*'> of i-o.k. oftrii a.s big as a Look, 
wlii"li havi." li^eii traii>|»«»rtf«l l»y glacuT-Lf. aipl liavr l»eeu I«H.lfp:d iu a promiiienc |HkjtitioB 
ill iilmnt-.v vall*"ys or hdvf Yn-en s*.dttr!f:Hl nver liilU ami plaiii>. An examination <rf 
tiif-irmiiit'i-alojirif.'al •Itaracti.T iiMrls to t)ie iileiitiHt-ation of tlieir stjurev ami, cons^|nentIr, 
t<^i the j»ath taken hy tho tran>iN^)rtiii^: ice. •Sr*.* 15<»ok III. Part II. Section ii. § 5.) 

Bain-waah— a loam or i*arth whii-li atraniulati's «iii thi' lower |iarts of ^loiies or at 
tlifir IiasJr, ail' I i>» diif tn tii»* ^iidual <.h'scent of the finer |Kirtit'lfs of ilisintegrated rocks 
hy t)ie ti-aii»iM>rtin^ artioii of rain. Urirk-earth is the name given in the south-cast 
of En;j:Ian<l to thick iiia.snf> of Mii.h hianu w)iir)i in extena^ively Uifed for making 
hiii.ks. 

Soil — the }iro<hi<'t of the siil^f rial de(.-omi»ositiou of riM-ks an(l of tlit* decay of jilanD 
anrl animals. I'l-imaiily thr character of t)ie .soil is determinKl hy that of the sul^ioiL 
of which indeed it is merely a further disintegi^ation. Aci-ording to tlie nature of th* 
ror-k iin«lerneath. a soil niav vaiv from a stitt' elav. throiii;h various davev and saudv 
I'Mims to nifie sau'l. The formation «»f snil is treated *}( in Book HI. Part II. Section 

II. § 1. 

Subsoil- -the hroken-ii]i {lurt of the n^-ks immeiliately under tht* soil. Its character. 
of c«»ui>e. is fletermined by that of the rcK-k out of which it is forme^l by subaerial di«in- 
t<';rratiijn. B<M)k III. l*:irt U. Sertiiiii ii. § 1. 

Blown Band -liMtsi* siind usually urran^'d in lines of dunes, fi-rmting a asaudy beach 
or in th** ari<I int«-iir»r <»f a continent. It is piled \ip hy the driving action of wind. 
(l>iH)k III. Tart II. Section i. It varies in eonijiosition, l>eiug sometimes entirely 
-Niliceon-*. as u|M»n ^hon-s wlu-n- >ilice«ms rocks are exiKiseil ; sometimes calcareous, where 
fh'rivi'd from trituraic<l shells. nuIliiMjrcs. or other calraitous organisms. The minute 
giains fri>m long-continut'il mutual friction assume i-emarkably i-ounded and iioliflhed 
fitiiiis. I^iyei> of Hner and marser {uirticles often alternate, as in water-formed sand- 
stone. On many fOii.st-lin«-.s in Euroiie, gi^as^es aiiil otlier ]ilant.> bind the surface of the 
shifting sand. The.s*.* layi-r'. of vegetation are apt to be eoverwl by fresh encroachments 
of the lo<»se material, and then ]»y their decay to give ris*,* to dark }ieaty seams in the 
sand. Caleai-e<iu» blown sand is «>om]>iU-te<l into hanl st<»ne by the a^-tion of rain-water, 
which alternately dis.so]vcs a little nf the linu*, an<l re-de}N)sits it on eva[)oration as a 
tliin cnist cementing the grains of siind together. In the llaliamas and Bermuda^ 
extensive masses of calcareous blown ^alnl have In-en cement***! in this way into solid 
stone, wliich weathers into pi«'tui-esiiue rrags and caves like a limestone of older geological 
datt-.' At Ni:w<iuay, (Cornwall, blown Sjind has Ikmmi by the decay of abundant land* 
shells scjliflitied into a mateiial capable of being used as a building-stone. 

Biver-sand, Sea-sand. — Wlim the rounded water-worn detiitiis is tinerthan tliat tu 
which the tfini gravel would be applied, it is called sand, though there is obriously do 
line to be drawn between the tw<i kinds of de]H.»sit, which necessarily graduate into each 
lit her. The ]iaiticles of .sand range down to such minute forms as can only be distinctly 
di.Merned with a mierof,i'opc. The smaller forms ai-e generally less well rounded than 
those of gr«'ater <liiiiensinns. no doubt l»ei'au^e their diminutive sijce alloa*s them to 
remain su>] Handed in agitated water, and tlius to escain* the mutual attrition to which 
the larger and heaviiri- grains aiv exi»ost»d ujhui the bottom. (Yivok III. Part II. Section 
ii.) S<t tar as exi^'rieiice has yet gone, theix' is no metluxl by which inorganic sea-sand 
can 1m! distinguishe<l from that of rivers or lakes. As a rule, sand consists lai^ly 



* For interesting aecoinits of the .Et^lian deposits of the Hahanias and Bermudas, 
Nel-on, V- •^- ^'•'"'- •^"- ix- !'• -00, Sir Wyville Thomson's *" Atlantic," vol. i. ; also J. 
J. Iteiu, .Sr/.'/.r/'//. A'«/. fi»>.>'fhrh. JWic/it. 1^69-70, p. 140, lS72-a, p. 131. On the Red 
Sands of the .Vrabiaii De.sert, svc J. A. Phillips, y. J. <f'tol. .*h-. xxxviii. (1882), |i. 110, alw) 
oyi. rif. XXX vii. (ISSl), p. I'J. 



PART 11 § vii FRAGMENTAL ROCKi^ 129 

(often wholly) of quartz-grains. The presence of fragments of marine shells will of 
course betray its salt-water origin ; but in the trituration to which sand is exposed on 
a coast-line, the shell-fragments are in great measure ground into calcareous mud and 
remove<i, 

^fr. Sorby has shown that, by microscopic investigation, nmch information may be 
obtaine<l regarding the history and source of sedimentaiy materials. He has studied 
the minute structure of modern sand, and finds that sand-grains present the following 
five distinct tyi>es, which, however, graduate into each other. 

1. Normal, angular, fresh- fonned sand, such as has been derived almost directly 
from the breaking up of granitic or schistose rocks. 

2. Well-woni sand in rounded grains, the original angles being completely lost, and 
the surfaces looking like fine ground glass. 

3. Sand mechanically broken into sharp angidar chips, showing a glassy fracture. 

4. Sand having the grains chemically corroded, so as to produce a peculiar texture of 
the surface, <liffering from that of worn gi'ains or crystals. 

5. Sand in which the grains have a perfectly crystalline outline, in some cases un- 
doubtedly due to the deiwsition of quartz upon rounded or angular nuclei of ordinary 
uon -crystalline sand.^ 

The same acute observer jwints out that, as in the familiar case of conglomei*atc 
pebbles, which have sometimes been used over again in conglomerates of very diiferent 
ages, so with the much more minute grains of sand, we must distinguish between the 
age of the grains and the age of the deposit formed of them. An ancient sandstone 
may consist of grains that had hardly been worn before they were finally brought to 
reut, while the sand of a motlem beach may have been ground down by the waves of 
many successive geological jieriods. 

Sand taken by Mr. Sorby from the old gravel terraces of the River Tay, was found to 
be almost wholly angular, indicating how little wear and tear there may be among 
particles of ([uartz jJxr of *ii mch in diameter, even though exi>osed to the drifting 
action of a rapid river.' Sand from the boulder clay at Scarborough was likewise 
ascertained to be almost entirely fresh and angular. On the other hand, in geological 
formations, which can be traced in a given du'ection for several hundred miles, a 
progressively large proportion of rounded particles may ])e detected in the sandy Ijeds, 
as Mr. Sorby luis found in following the Greensand from Devonshire to Kent. In wind- 
blown sand exposed for a long period to drift to and fro along the surface the larger 
particles and pebbles ac<[uire a remarkably smoothed and polished surface. 

The occurrence of various other minerals besides «[uartz in ordinary sand has long 
been recognised, but we owe to the recent observations of Mr. A. B. Dick the discovery 
that among these minerals some of the most plentiful and most perfectly jjreserved 
belong to species that were not supposed to be so widely dilTused, such as zircon, rutile, 
and tourmaline. He has found that these heavy minerals constitute sometimes as much 
as 4 per cent of the Bagshot sand of the older Tertiary series of the London basin. ^ 
Felspuv, micas, hornblendes, x>yroxenes, magnetite, glauconite and other minerals may 
likewise be recognised. The remarkable perfection of some of the crystallographic 
forms of the minuter mineral constituents of certain sands has been well shown by 
Mr. Dick. 

Varieties of river or sea -sand may be distinguished by names referring to some 
remarkable constituent, e.g.y magnetic sand, iron-sand, gold-sand, auriferous sand, &c. 

GntTVl, Shingle — names applied to the coarser kinds of rounded water- worn detritus. 

* Address, Q, J. Geol. Soc. xxxvi. (1880), p. 58, and Monthly Microscop. Journ. Anniv. 
Address, 1877. 

s See Book UI. Part II. Section u. § iii. 

» Xaiurt^ xzxvL (1887), p. 91, Mem, Otd, Surv. 'Geology of London,' vol. i. (1889), 
p. 523. Teall, 'Microscopic Petrography,' Plate xliv. 

K 



1 30 GEOGNOH Y book n 

In Gravel, the avera^f size of tlie conipoiieiit ]H;1>ble.s rang4.>s from that of a small ]jea 1^) 
to al)Out that of a walnut, thou«(Ii of course many included fragments mill be obserred 
which exceed these limits. In Shingle, the stones are coarser, ran^ng up to blocks at 
big as a man's head or larger. Oennan geologists distinguish as ''sehotter/' a shingle 
containing dis]>erse<i Ix^uldci's, and ''scliotter- conglomerate," a rock wherein these 
materials have become consolidated.' All these names are apiilie<l quite irresiiective of 
the comjiosition of the fragments, which varies greatly from |X)iiit to jtoiut. As a rule, 
the stones consist of hanl rocks, since these are 1>est fitted to withstand the i»owerfal 
grinding action to which they are expose<l. 

Conglomerate (Puddiugstone) -a roi'k formed of consolidated gravel or shin^e. 
The com]K)nent jiebbles arc rounded and water-worn. They may consist of any kind 
of rock, though usually of some hard and durable sort, such as quartz or quartzite: 
A special name may be given according to the nature of the i)ebbles, as quartz-eon* 
glomerate, limcstone'Conglomi>rate, granite-conglomerate, &c., or according to that of 
the paste or cementing matiix, which may consist of a hardened sand or clay, and 
may be siliceous, calcareous, argillaceous, or femiginous. In the coarser conglomerates, 
where the blocks may exceed six feet in length, there is often very little indication 
of stratification. Except where the Hatter stones show by their general parallelism the 
rude lines of d('i»osit, it may !>«• only when the mass of conglomerate is taken as ■ 
whole, in its relation to the rocks l>elow and above it, that its claim to be considered 
a l)edded rock will Ki conceded. ^ The occun^ence of (K'casional bands of conglomerate 
in a series of arenaceous strata is analogous probably to that of a shingle-bank w 
gi'avel-beach on a mo<lern coast-line. Hut it is not easy to undei-stand the circom- 
stances under which some ancient conglomerates accumulated, such as that of the 
Old Red Samlstone of Central Scotland, which attains a thickness of many thousand 
feet, and consists of well-rounded and smoothed blocks often several feet in diameter. 

In many ol«l (ronglomerates (and even in those of Miocene age in Switzerland) tbe 
conqKment ]K;bbles may be observed to have indented each other. In such cases alio 
they may be found elongated, distorted or split and recementiMl ; sometimes the sanw 
I>ebble has l>een cnished into a number of piece>, which are held together by a retaining 
cement. These phenomena \m\\t to great pressure, and some internal ivlative movement 
in the rocks. (Book III. Part I. Section iv. § 3.^ 

Breccia — a rock coni]>ost.Ml of angular, instead of rounded, fragments. It commonly 
prc*scnts less trace of stratification than conglomerate. Inteniiediate stages between 
these two rocks, where the stones ai-e |>artly angular and ]tartly sultangular and rounded, 
are known as hrecciatcd coivjlomrrati'. Considered as a detrital deposit formed by 
buiKjrficial waste, breccia |K)ints to the dlsint^'gration of rocks by the atmosphere, and 
the accumulation of their fragment:» with little or no intervention of numiug water. 
Thus it may be formed at the base of a elilf, eitlK*r subaerially, r>r where the debris of 
the cliff falls at once into a lake or into deep sea-water. 

The term Breccia has, however, been applieil to i-ocks formed in a totally diflforent 
manner. Angular bl<K*ks of all sizes and shapes have lieen dischargdl from volcanic 
orifices, and, falling liack, have coiLsoIidated there into masses of breeciated material 
(volcanic breccia). Intrusive igneous eruptions have sometimes torn off fragments of the 
rrkcks through which they have ascended, and these angular fragments have been 
encIose<l in the liquid or }>asty mass. Or the intrusive i*ock has cooled and solidified 
externally while still mobile within, and in its ascent has caught uj) and involved some of 
these consolidattni ]>ails of its own substance. Again, where solid masses of rock within 
the crust of the earth have giound against each other, as in dislocations, angalar frag- 
mentary rubbish has been ]»nxluced, which has subsequently been consolidated by some 
infiltrating cement (Fault-rock). It is evident, however, that breccia formed in one or 



* See, for exunq.'le, an account of the sehotter-conglomerates of Northern Persia by 
E. Tietze, JiUirh. GeoL KeichsariAf. Vienna, 1881, p. 68. 



PART n § vii FRA OMENTA L ROCKS 131 

other of these hyx)ogene ways will uot, as a rule, be apt to be mistaken for the true 
breccias, arising from superficial disintegration. 

ftfinilTtinift (Or^) ^ — a rock composed of consolidated sand. As in ordinary modern 
sand, the integral gi-ains of sandstone are chiefly quartz, which must here be regarded 
as the residue left after ail the less durable minerals of the original rocks have hecu 
carried away in solution or in suspension as fine mud. The colours of sandstones arise, 
not so much from that of the quartz, which is commonly white or grey, as from the 
film or crust which often coats the grains and holds them together as a cement. Iron, 
the great colouring ingredient of rocks, gives rise to red, brown, yellow, and green hues, 
according to its degree of oxidation and hydration. 

Like conglomerates, sandstones differ in the nature of their component grains, and 
in that of the cementing matrix. Though consisting for the most part of siliceous 
grains, they include others of clay, felspar, mica, zircon, rutile, tourmaline, or other 
minerals such as occur in sand (p. 129), and these may increase in number so as to give a 
special character to the rock. Thus, sandstones may be argillaceous, felspathic, mica- 
ceoQB» calcareous, kc. By an increase in the argillaceous constituents, a sandstone may 
pass into one of the clay-rocks, just as modern sand on the sea-floor shades imperceptibly 
into mud. On the other hand, by an augmentation in the size and sharpness of the 
grains, a sandstone may become a grit, and by an inci-easo in the size and numWr of 
pebbles, may pass into a pebbly or conglomeratic sandstone, and thence into a tine 
conglomerate. A piece of fine-grained sandstone, seen under the microscoxte, looks like 
a coarse conglomerate, so that the difference between the two rocks is little more than 
one of relative size of particle.^. 

The cementing material of sandstones may be ferruginoua, as in most oixlinaiy red 
and yellow sandstones, where the anhydrous or hydrous iron -oxide is mixed with clay 
or other impurity — in red sandstones the grains are held together by a hjematitic, in 
yeUow sandstones by a limonitic cement ; argillaaoiiSy where the gi*ain8 arc united 
by a base of clay, recognisable by the earthy smell when breathed upon ; calcareous, 
where carbonate of lime occurs either as an amorphous paste or as a ciystalline cement 
between the grains ; siliceous, where the component particles are bound together by 
silica, as in the exposed blocks of Eocene sandstone known as " greyweathers " in 
Wiltshire, and which occur also over the north of France towards the Ardennes. 

Among the varieties of sandstone the following may here Ikj mentioned. Flag- 
stone — a tliin-bedded sandstone, capable of being split along the lines of stratification 
into thin beds or flags ; Micaceous sandstone {mica-psammitc) — a rock so full of 
mica-flakes that it splits readily into thin lamime, each of which has a lustrous surface 
from the quantity of silvery mica. This rock is called "fakes" in Scotland. Free- 
stone — a sandstone (the term being apjilied sometimes also to limestone) which can Ijc 
cat into blocks in any direction, without a marked tendency to split in any one plane 
more than in another. Though this rock occurs in beds, each bed is not divided into 
laminae, and it is the absence of this minor stratification which makes the stone so useful 
for anshitectural purix)ses (Craigleith and other sandstones at Edinburgh, some of which 
contain 98 per cent of silica). Glauconitic sandstone (green -sand) — a sandstone 
containing kernels and dusty grains of glauconite, which im[)arts a general greenish hue 
to the rock. The glauconite has probably been deposited in association with decaying 
organic matter, as where it fills echinus-spines, foraminifera, shells anrl corals on the 
floor of the present ocean. ^ Buhrstone— a highly siliceous, exceedingly compact, 
thong^ cellular rock (with Chara seeds, &c. ), found alternating with unaltered Tertiary 
strata in the Paris basin, and forming from its hardness and roughness, an excellent 

* See J. A. Phillips on the constitution and history of grits and sandstones. Quart. 
Joum. Gtol. Soe, xxxvii. (1881), p. 6. For analyses of some British sandstones used as 
building stones, see WaUace, Proc. Phil. Sot: Olasguic, xiv. (1883), p. 22. 

^ Ante, p. 77 ; Sollas, Geol. Mag. iii. 2nd ser. p. 539. 



132 tfEOf.L\n.<y DOOKU 



iiiatcrial f«ir the p-irnUtnins of tluur-iiiills. iimy U* unriitioiietl ben*, tliouj^rh it jirobablj 
liiLs \Htfn fririii«-d by tbtr prt-i-iiiitatioii iif silira tliroiij;b tbe action of or^iiisms. Arkose 
'jranUif sintd.'*fonr)—a. io«-k comjiosc^l of ilisiiitt-gratitl granite, and fouDd in geolqgifial 
lorinations of ditfen^nt a^^'s, wbirh liave In-eii di-rivni fn)m granitic rocks. Crystallized 
sand'itono— an arena'-fous r«M;k in \vbi<.b a d^Hxit of rr}'stalUiii.> quartz lias taken place 
upon tbe individual grains, cacli n{ wliirb lie^'oiues tlic nucleus of a more or less perfect 
•juaitz crystab Mr. S«irby bas olrsi-rviil sucb crystalliziMl sand in de|H)«its of variom 
ag»'s from tbe Oolites down to tbe Old Ked Sandstone. * 

Qreywacke — a r.-nni{>a<:t aggregati' of roundel or sul^ngular grains of quartz, fel8]iAr, 
slate, or otlier miueral.s or rock^, cem»'nted by a jiaste wliicb is usually siliceous, but 
may 1m- argillaceous, feNjiatbic, calcareous. t»r antbracitic (Fig. 21). (^rey, as its name 
denotes, is tbe prevailing colour : but it {msses into brown, brownisb-]iurple, and ifomt- 
times, wbere antbi-acite ]ire<lominates, into black. Tlie nxrk is distinguished from 
onlinary sandstone by its darker Ime, its bartlness, tbe vanety of its comiionent graln», 
ami, above all. by tbe comjiact cement in wbicb tbe grains are iml^eddod. In many 
varinties, so jKjrva'led is tbe vovk by tbe silice<ms j^ste, tbat it [K>sse.sses great toiigbnesi, 
and its gi-ains seeTu to graduate into eacb otber as well as into tbe surrounding matrix. 
Sui-b rocks wben tine-gi*ained, can banlly, at first sigbt or witb tbe unaided eye, l>e di»« 
tingiii»be<l from some eom{iti«.-t igneous r<X'ks, tbougb a micms^ropic examination at onoe 
r«^ veals tbeir fragmi-nlal fbai-aj'ter. In otber cas«\s, wbeiv tbe greywacke has been fonned 
mainly out of tbe debriji of granite, quartz - ]X»rpbyry, or otber feLs]tathie masses, the 
grains eonslst so largely of felsjiar, and tbe jiaste aUo is so felsjiathic, that the rotk 
migbt 1k! mistaken for sonu* close-grained granular jH^rjibyry. Greywacke occurs exten* 
».ively among tbe Pabcozoie formations, in IkmIs alternating witb shales and conglo* 
meratcs. It ivpresents tbe nniddy s-uhI of some of tbe Palfeozoic sea-floors, retaining often 
its n]>])le-marks and sun-eracks. Tbe metaniorpbism it bas undergone has generally 
not IxMMi great, and for tbe most i>art is limite<l to induration, ]tartly by pressure and 
I tartly by ]ienneation of a siliceous cement. But wbere felspatbic ingredients pre^'ail, 
tbe roik bas offered facilities for alteration, and bas l>een here and there clianged into 
highly crystalline mica-scbists full of garnets and other secfuidary minerals (contact- 
meiamf»rpbism at tbe granite of Xew Galloway, Scotland, jiostea, j). 606). 

Tbe mon? fissile fnn'-giained vaiieties of this i"ock have been temied greywacke-slate 
(p. l'3r>). In these, a> well as in givwacke, organic remains occur among the Silurian 
ami iK'Vonian formations. Sometimes in the I^wer Siluiian rocks of Scotland, these strati 
become black witb carl)ona<-eous matter, amoug which vast numl>ers of graptoUtes may 
In.' obs<Mved. Gradations into sandstone are termed Greywacke-sandstone. In Nor- 
way tbe re<ldish felspatbic greywacke or sindstone of tbe Primordial itx^ks, is called 
Sparagmite ; similar material forms much of tlnj Toiridon sandstone of Scotland. 

Quartzite. -An altered siliceous sandstone (siMt p. 180\ 

2. Clay Bocks (Pelites). 

TbcMt arc ci>nq»osed of line argillaceous se<liment or mud, derived from the waste of 
rocks. Perfectly pure clay or kaolin, bydi-atiMl silicate of alumina, may be obtained where 
granites and other fels|iar- bearing rocks decom^Ktse. But, as a nile, the alliaceous 
materials are mixi^d witb various impurities. 

Clay, Mud. — Tbe decom|K)sition of felsiars and allie<l minerals gives rise to the 
formation i>f hydrous aluminous silicatt^s, which occurring usually in a state of fine sub- 
tlivision, are (ra)iable of lH;ing held in sus]>ension in water, and of being transported to 



' <j. J. fi'of. SfH-.. XXX vi. i». 63. See Daubrec, Ann. tf'S Mi/ira, 2nd ser. i. p. 206. A. 
A. Young, A nun: J fit in. S:i. 'Srd ser. xxiii. 257 ; xxiv. 47, and es))ecially the work of 
Irving and Van Hise (quoted on ]i. 110), which gives some excellent figures of enlaiiged quaiti- 
grains. 



PAKT II § vii FBAGMENTAL ROCKS 133 

great distances. Tliese substances, differing much in com^wsitiou, are embraced under 
the general term Clay, which may be defined as a wliite, grey, brown, red, or Iduish 
substance, which when dry is soft and friable, adheres to the tongue, and shaken in 
water makes it mechanically turbid ; when moist is plastic, when mixed with much 
water becomes mud. It is evident that a wide range is i>ossi]>le for varieties of this 
substance. The following are the more imjxirtant. 

Kaolin (Porcelain -clay, Chinan-lay) has l»een already noticed (]>. 77). 

Pipe-Clay — white, nearly pure, and free from iron. 

Flre-Clay — largely found in connection with coal-seams, contains little iron, and 
is nearly free from lime and alkalies. Some of the most typical fire-clays are those long 
uaed at Stourbridge, Worcestershire, for the manufacture of jiotterj'. The best glass- 
house ]iot-clay, that is, the most refractory, and therefore used for the constniction of 
pots which have to stand the intense heat of a glass-house, has the following comi>osi- 
tion : silica, 73*82 ; alumina, 15*88 ; protoxide of iron, 2*95 ; lime, trace ; magnesia, 
tZBce ; alkalies, *90 ; sulphuric acid, trace ; chlorine, trace : water, 6*45 ; sjiecific 
gravity, 2*51. 

Qannister — a very siliceous close-grained variety, found in the Lower Coal measures 
of the North of England, and now largely ground down as a material for the hearths of 
iron fnmaces. 

Brick-day — j>ro])erly rather an industrial than a geological tenn, since it is ap])lied 
to any clay, loam, or earth, from which bricks or coarse j)otteiy are made. It is an iin- 
\mre clay, containing a good deal of iron, with other ingi'edients. An analysis gave the 
following composition of a brick -clay : silica, 49*44 ; alumina, 34*26 ; sewpiioxideof iron, 
7*74 ; lime, 1*48, magnesia, 5*14 ; water, 1*94. 

Fuller*! Earth (Terre a foulon, Walkerde) — a gieenish or brownish, earthy, soft, 
somewhat unctuous substance, with a shining streak, which does not iKH'ome plastic 
with water, but cnimbles down into mud. It is a hydrous aluminous silicate with some 
magnesia, iron-oxide and soda. The yellow fuller's earth of Keigate contains silica 44, 
alumina 11, oxide of iron 10, magnesia 2, lime 5, soda 5.^ In England fuller's earth 
occurs in beds among the Jurassic and Cretaceous formations. In Saxony it is found as 
a result of the decom}x>sition of diaba.se and gabbro. 

Wacke — a dirty -green to bro\%'ni8h - black, earthy or comi)act, but tender and 
apjiarently homogeneous clay, which arises as the ultimate stage of the deconipasition 
of basalt-rocks in situ. 

Loam — an earthy mixture of clay and .sand with more or less organic matter. The 
black soils of Russia, India, &c. (Tchemosem, Rcgur), are dark deposits of loam rich in 
organic matter, and sometimes upwards of twenty feet dee]». 

Loan — a pale, somewhat calcareous clay, probably of wind-drift origin, found in 
some river- valleys (Rhine, Danube, Mis.si.ssippi, &c.), and over wide regions in China and 
elsewhere. It is described in Book III. Part II. Sect. i. § 1. 

Laiarite — a cellular, reddish, ferruginous clay, found in some tropical countries as 
the result of the subaerial decomi^sition of rocks ; it accpiires great hardness after being 
c^narried out and dried. 

Till, Bonldtr-day — a stiff sandy and stony clay, varying in colour and com|>osition, 
according to the character of the rocks of the district in which it lies. It is full of 
worn stones of all sizes, up to blocks weighing .several tons, and often well -smoothed and 
striated. It is a glacial dejwsit, and will be described among the fonnations of the 
(Uacial Period. 

Muditone — a fine, usually more or less sandy, argillaceous rock, having no fissile 
character, and of somewhat greater hardness than any form of clay. The term Clay- 
rock has lieen applied by some ^mters to an indurated clay recjuiring to be ground and 
mixed with water l)cfore it acquires plasticity. 

» Ure's Diet. Arts, &c. ii. p. 142. 



1.34 GEOGNOSY book u 

Shale (Schihte, Sdiieferthon) — a general term to describe clay that has assumed a 
thinly stratified <»r fissile structure. Under this term are included laminated and some- 
what haitlened ar^pllaeeous roc-ks, which an> ca]>a1>le of being split along the lines of 
de^josit into thin leaves. They |»n«ent almost endle^ss varieties of texture and compofd- 
tion, ])assing, on the one hand, into clays, or, where much indurated, into elates and 
argillaceous schists, on the other, into flagstones and sandstones, or again, throngh cal- 
careous gradations iuUt limestone, or through ferniginous vaiieties into clay-ironstone, 
and through bituminous kinds into coal. 

Clay-slate (Sehiste ardoise, Thons^-hiefer). — Under this name are included certain 
hanl fissile argillaceous masses, composed primarily of comjiact clay, sometimes with 
megascopic and mierosco])ic si^ales of one or more micaceous minerals, granules of quartz 
and cubes or concretions of ])yritcs, as well as veins of quartz and calcite. The fissile 
structure is sjiecially characteristic. In some vtaiea this structure coincides with tliat of 
oiiginal dejKtsit, as is proved by the alternation of fissile beds with bands of hardened 
sandstone, conglomerate or fossiliferous limestone. But for the most part as the rocks 
have l)ei>n much comxtressed, the fissile structure of the argillaceous bands is inde|)endent 
of stratification, and can be seen traversing it. Sorby has shown that this superinduced 
fissility or "cleavage" has resulted from an internal rearrangement of the particles in 
l>lanes ])er})endieular to the direction in which the rocks have Ix^en compressed (see Book 
III. Part I. Section iv. § 3). In England the tenn "slate " or "clay-slate '* is given to 
argillaceous, not obviously crystalline rocks |K)ssessing this cleavagc-stnioture. Where 
the micaccims lustn? of the finel}' disseminat^-d superinduced mica is prominent, the rocks 
are f»hyllites. 

Microscopic examination shows that while some argillaceous rocks consist mainly of 
gi-anular debris, many cleaved clay-slates contain a large proportion of a mioaceous 
mineral in extremely minute flakes, which in the best Welsh slates have an averags 
si/e of Yihio of an inch in bi*eadth, and haVu o{ an inch in thickness, together ¥rith ¥eiy 
fine black hairs which may l>e magnetite.' Moveover, many clay-slates, though to 
outward ap[)earance thoroughly noncrystalline, and evidently of fragmental com|K>8ition 
and serlimentary origin, yet contain, sometimes in remarkable abundance, microscopie 
microlites and crystals (»f different minerals placed with their long axes iiarallel with the 
planes of fissility. These minute bodies include yellowish -brown needles of rutile, 
greenish or yellowish flakes of mica, scales of calcite, and probably other minerala' 
Small granules of quartz containing fluid-cavities, show on their surfaces a distinct 
blending with the substance of the surrounding rock. M. Kenaitl has found that the 
Belgian whet-slate is full of minute crystals of garnet.^ Some of the more ciystaUine 
varieties (phylliti') are almost wholly comi»r»sed of minute crystalline ])articles of mica, 
({uartz, fels]tar, chlorite, and rutile, and form an intcnnediate stage between ordinaiy 
clay-slate and mica-schist. 

A distuiction has bt?en drawn by sonu< i>etrographei-s between cei'tain rocks (phyllitCi 
Urthonschiefer) which occur in Archaean regions or in gi'oujw probably of high antiquity, 
and others (ardoise, Thonschiefer) which are found in Palreozoic and later formations. 
But there does not ap]Myir to be adi^quate justification for this grouping, which luw prob- 



* Sorby, Q. J. Geol. »^i«'. xxvi. p. 68. 

' These " clay-slate needles *' were j^robably not crystallized couteniixjraueously with the 
deposit of the original rock. In some cases they may have been de]>osited with the rest of the 
sediment as part of the debris of pre-existing crystalline rocks (see p. 129) ; but in general 
they appear to have Iteen developed where they now occur by subsequent actions (see 
/nostra J p]i. 312, 545). For their character see Zirk el, * Mik. Beschaff.' p. 490. Kalkowsky, 
y. Juhrh. 1870, p. 382; A C'athrein, np, dt. 1882 (i.), p. 169. F. Peuck, Siisb. Bayer. 
Akfuf, Math. PInjs. 1880, p. 461. A. Wichniann. Q. J, Otol. Stn: xxxv. p. 156. 

'^ Aauf. Roy. Btlf/iqne^ xli. (1877). See also his paper on the composition and stractare 
of the phyllades of the Ardennes, BuU. Mns. Roy. Bfly. iii. (1884), p. 231. 



PART u § vii FRAGMENTAL ROCKS— VOLCANIC 136 

ably been suggested rather by theoretical exigences than by any essential differences 
between the rooks themselves. That the whole of the series of argillaceous rocks, begin- 
ning with clay and passing through shale into slate and phyllite, is of sedimentary 
origin is indicated by the organic remains, false bedding, ripple-mark, &c., found in 
those at one end of the series, and by the insensible gradation of the mineralogical 
characters through increasing stages of metamorphism to the other end. Some micro- 
scopic crystals may possibly have been originally formed among the muddy sediment on 
the sea-floor (see p. 459). Others may have formed part of the onginal mechanical 
detritus that went to make the slate! But, for the most part, they have been subsequently 
developed within the rock, and represent early stages of the process which has culminated 
in the production of crystalline schists. The development of crystals of chiastolite and 
other minerals in clay-slate is frequently to be observed round bosses of granite, as 
one of the phases of contact-metamorphism (see pp. 568, 605). 

A number of varieties of Clay-slate are recognised. Roofing slate (Dachschiefer) 
includes the finest, most compact, homogeneous and durable kinds, suitable for roofing 
hooses or the manufacture of tables, chimney-pieces, writing-slates, &c. ; it occurs in the 
Silnrian and Devonian formations of Ontral and AVesteni £uro()e. Anthracitic- 
slate (autliracite-phyilite, alum-slate), dark carbonaceous slate with much irou-disul- 
phide. Bands of this nature sometimes run through a clay-slate region. The carbon- 
aceous material "arises from the alteration of the remains of ])lants (fucoids) or animals 
(freqneutly graptolites). The marcasite so abundantly associated with these organisms 
decomposes on exposure, and the sulphuric acid produced, imiting \rith the alumina, 
potash, and other bases of the surrounding rocks, gives rise to an efflorescence of alum, 
or the decomposition produces sulphurous springs like those of Moffat. The name Grey- 
wacke- slate has been applied to extremely fine-grained, hard, shaly, more or less 
micaceous and sandy bandvS, associated with greywacke among the older Palaeozoic 
rocks. Whet-slate, Novaculite, Hone-stone, is an exceedingly hard fine-grained 
siliceous rock, some varieties of which derive their economic value from the presence of 
microscopic crystals of garnet. The various fonns of altered clay -slate are described at 
p. 179 among the metamorphic rocks. * 

Poroellmnite (Argillite) or baked shale — a name applied to the exceedingly indurated 
sometimes partially fused condition which shales arc apt to assume in contact with 
dykes and intrusive sheets or bosses. For an account of this form of contact-meta- 
morphism see p. 600. 

3. Volcanic Fragmental Bocki — Toffs. 

This section comi>rises all deposits which have resulted from the comminution of 
volcanic rocks. They thus include (1) those which consist of the fragmentary materials 
ejected from volcanic foci, or the true ashes and tuffs ; and (2) some rocks derived from 
the superficial disintegration of already erupted and consolidated volcanic masses. 
ObTioasly the second series ought properly to be classed with the sandy or clayey rocks 
aboTC described, since they have been formed in the same way. In practice, however, 
these detrital reconstructed rocks cannot always be certainly distinguished from those 
which have been formed by the consolidation of true volcanic dust and sand. ' Their 
chemical and lithological characters, both megascopic and microscopic, are occasionally 
so similar, that their respective modes of origin have to be decided by other considera- 
tions, such as the occurrence of lapilli, bombs, or slags in the truly volcanic series, and of 
well water-worn pebbles of volcanic rocks in the other. Attention to these features, 
however, usually enables the geologist to make the distinction, and to perceive that the 
number of instances where he may be in doubt is less than might be sup})osed. Only a 
comparatively small number of the rocks classed here are not true volcanic ejections.^ 

' For a classification of tuffs and tuffaceous deposits see E. Reyer, Jahrh, K. K. deol, 
ReUhaantt, xxzL (1881), p. 57. 



1 3(5 aKOGXOS y BOOK 11 



Roferriiig to the account of volcanic action in Hook III. Part 1. Sect, i., we may here 
merely define the use of the names hy which the diflerent kinds of ejected volcanii- 
materials are known. 

Volcanic Blocks — angular, sub-angular, round, or irregularly - shaped masses of 
lava, several feet in diameter, sometimes of uniform texture throughout, as if they were 
large fragments dislodged by explosion from a previously eon.soIidated rock, sometimes 
compact in the intcnor and cellular or slaggy outside. 

Bombs— round, elliptical, or discoidal i»ieces of lava from a few inches up to one 
or more feet in diameter. They are freipiently cellular internally, while the outer iiarts 
arc fine-grained. Occasionally they consist of a mere shell of lava ^vith a hollow 
interior like a bomb-shell, or of a easing of lava enclosing a fragment of rock. Their 
mofle of origin is explainwl in IJook III. Pait I. Sect. i. § 1. 

Lapilli (rapilli) — ejected fragments of lava, round, angidar, or indetinite in shape, 
var}'iug in size from a i^ea to a walnut. Their minei-alogical composition dejiendfl ujion 
that of the lava from which they have been thrown up. Usually they are (Kirous or 
finely vesicular in texture. 

Volcanic Band, Volcanic Ash— the finer detritus enipted from volcanic orifices, 
consisting jMii-tly of rounded an<l angular fragments up to alwut the size of a ]ieft 
derived from the explosion of lava within eruptive vents, |>artly of vast quantities 
of microlitcs and ciystals of some of the minerals of the lava. The finest dust w in a 
state of extremely minute sulidi vision. When examined un<ler the niicrosco])e, it ii 
somethnes found to consist not only of minute ciystals and mici'olites, but of volcanic 
glass, which niaj- be ob.servcd adhering to the microlites or ciystals round which it 
flowed when still ^Mirt of the fluid lava. The i)resence of minutely cellular fragments is 
characteristic of most vok-anic fragmental rocks, ami this structure may commonly be 
ol.)scr\*ed in the mii-roseopic fragments and filaments of gla.'ts. 

"When these various materials are allowed to accumulate, they become consolidated 
and receive distinctive names. In cases where they fall into the sea or into lakes, they 
are liable at the outer margin of their area to be mingled with, and insensibly to {«■ 
into onlinary non-volcanic sediment. Hence we may ex])ect to find transitional varieties 
between rocks fornie«l directly from the results of volcanic ex})losion and ordinary sedi- 
mentary de]>osits. 

Volcanic Conglomerate — a roitk comi>osed mainly or entirely of rounded or sab- 
angular fragiuents. chiefly or wholly of voUymic rocks, in a |>aste of the same materialft, 
usually exhibiting a stratified arrangement, and often found intercalated l>etH'eeii 
successive sheets of lava. Conglomerates of this kind may have l^een formed by the 
accumulation of rounded materials cjectetl from volcanic vents ; or as the result of the 
aqueous erosion of previously solidified lavas, or by a combination of both these processes. 
Well-rounded and smoothed stones almost certainly indicate long-continued water-action, 
i-athev than trituration in a volcanic vent. In the Western Territories of the United 
States vast tracts of countiy are covei-ed with masses of such conglomerate, some- 
times 2000 feet thick. Ca])tain Dutton has shown that similar dei)Osits are in eourse 
of fonnation there now, merely by the infhu^nce of disintegration uiKin exjKMed 
lavas. * 

Volcanic conglomerates receive difhirent names acconling to the nature of the com- 
]ionent fragments : tints we have hastiIt-eoHfjIomcratc,% where these fragments are 
wholly or mainly of basalt, fmcIiyti-confjIomeratcSf jwrjthifritt.-couglojnerateSf pJwtiolite- 
t'onglomcratcs, &c. 

Volcanic Breccia resembles Volcanic Conghmierate, except that the stones are 
angular. Tliis angularity indicates an absimce of atiueous erosion, and, under the 
circumstances in which it is found, usually ^Ktints to immediately adjacent volcanic 
explosions. There is a great variety of breccias, as ftamft-birccia, duiltasc-brrccia^ kc„ 

' * High IMateaux of Utah,* p. 77. 



vii FRAGMENTAI. ROCK>>— VOLCANIC 137 

lio ACElomenLte — a tumultuous assemblage of blocks of all siz«a up to nissseH 
lids in dismeler, met with in the "neoks" or jitjies of old volcanic oritices. 
■a uid paste are commonly of one or wore volcanic rocks, such as felsite, 
1 or basalt, but tliey include also fragments of the surrounding rocks, whatever 

ba, through which the volcanic orifice has been drilled. As a rule, agglomerate 
of stratificatioii ; but Hometimes EC includes |iortions wliich hare a more or less 
rangement into beds of coarser and finer detritus, often placed on end, or inclined 
<t directions at high angles, as described in Book IV. Part Vll. Sect. i. % 4. 
llo Tuff. — This general term may be made to include all the finer kinds of 
letritos, ranging, on the one hand, through coarse gravelly dei-osits into con- 
«, and on the other, into exceedingly coni|>act fino-grained rocks, formed of the 
1 moat impalpable kind of volcanic dust. Some modern tuffs are full of 
, derived from the lava which was blown into dust. Others are formed of 
Dded or angular grains of different lavas, n-itb fragments of various rocks 
rhioh the volcanic funnels have been drilled. The tuffs of earlier geological 
kTB often been so much altei'ed, that it is difficult to state what may Imvo I)ocn 
inal condition. The alraence of micro- 
gUas in tbeni is no [iroof that they are 
toOii ; for the preaence of these liodies 
ipoD the iiatnre of the lavas. If the 
re not vitreous and microlitie, neither 

the toffs derived from theiu. In the 
mu volcanic area of Central Scotland, 
m made u]> of debris and blocks of the \ 
ata, and, like these, arc not microlitie. 
I wme places they abound in fragments 
■ic glass called jialagonitc. (Kig- 2.1. 
, p. 138,) 

bave consolidated sometimes under 
netimes on dry land. As a rule, they 
etiy stratified. Near the original vents 

in they commonly present rapid alter- ^*'jfc^~"^^™X»TlIffC.rBumtr.^!^!!" 
'. Goer and coarser detritus, indicative of fj(( 
1 phases of volcanic activity. They 

y shade off into the sedimentary tonnationa with which they were con- 
eons. Thus, we have tuffs passing gradnally into shale, limestone, sand- 
Tlie iiitennediate varieties have been called laiiy ahalc, tuffuctoiiH dwir, 
(ujTj Ac. Kruui the circumstances ot their formation, tuffs freiinently pi'eserve 
ns of plants and animahi, both terrestrial and aquatic. Those of Muute Somma 
:agments of land-plants and sliellK. Some of those of Carlioniferons age in 
Mtland have yieliled crinoids, brachiojiods, and other marine organisms. Like 
Ingmentary volcanic rocks, the tuffs may be subdivided according to the nature 
va from the disintegration of which they have lieen formed. Thus we have 
i, irtKhyU-tiiffs, bmnll-hif*, piiiiiice-tiiffii, poriiliyrite-tHfa, 4c. A fen- varieties 
lal characteristics UMy lie mentioned here.' 

— « iiale yellow or grey rock, rough to the feel, comi«aed of an earthy or 
ptuniceous dust, in nliith fragments of pumice, trachyte, greyuaike, basalt, 

th« Dccarreuce and structure of tuffa, sea J. C. Ward, Q. J. Ifeol. -Sor. iixl. p. 
er, Jahri. Oenl. Ueich^ansl. 1881, j^ 57 ; Geikie, Tmnt. /l-fi. &"-. Jldiii. xxix. ; 
, Z. Ufiilark. tlent. 0\m. x\\y. ]•. .'liS ; Peuck, o/.. n'l. xxxi. p. 504. Oo tbe 
s of Scania, K. Eichstadt, .•h-eri.jrs Ueol. CHil-rtekii, ser. c. So. E8 (1883). On 
aoiphism of tuffii into luva-hke rorks, see lliitton's 'Hi^li llnteaui of Utah' 
Dgraph. and Geol. Survey of Itocky Mounts.), 1880, p. 7U. 




138 GEOGNOSY book u 



carbonized wood, &c., are imbedded. It has tilled up some of the valleys of the Eifel, 
where it is largely <{uarried as a hydraulic mortar. 

Peperino — a dark-brown, earthy or granular tuff, found in considerable quantity 
among tho Alban Hills near Rome, and containing abundant cr^'stals of augite, 
mica, leucite, magnetite, and fragments of ciystalline limestone, basalt, and leucite- 
lava. 

Palagonite-TofF — a Wddod aggregate of dust and fragments of Ijasaltic lava, among 
wliich ai*e conspicuous angular pieces and minute granules of the i»ale yellow, green, red, 
or brown basii; glass called jwilagonite. This vitreous substance is intimately related to 
the basalts (p. 1 72). It ap^x^ars to have gathered within volcanic vents and to have been 
emptied thence, not in streams, but by successive aeriform explosions, and to ha\'e been 
subsefpiently more or less altiTKl. The percentage comjKtsition of a s|)ecimcn from the 
typical locality, Palagonia, in the Val di Noto, Sicily, was estimated by Sartorius vou 
"NValtershausen to be: silica, 41*26; alumina, 8*60; feme oxide, 25*32; lime, 5*59; 
magnesia, 4*84 ; i^itash, 0*.'»4 ; soda, 1*06 : wat«T, 12*79. This nwk is largidy developed 
among the ju'txlucts of the Icelandic and Sicilian V(»lcanoes ; it occurs also in the £lfel 
and in Nassau. It has been found to be one of the characteristic features of tnffs of 
CarlK>niferous age in Central Scotland ^ (Fig. 23). 

Schalstein. — Tnder this name, Gennan jietrographei's have placiMl a variety of green. 
grey, rcil, or mottled fissile rocks, impregnat<'«i with carbonate of lime. They are inter- 
stratitied with the Devonian formations of Nassau, the Ilarz and Devonshire, and with 
the Silurian rocks of Bohemia. They sometimes contain fragments of clay-slate, and 
are <K'casionally fossil ifenms. They prejicnt amygdaloidal and porphyritic, as well as 
IK?rfectly laminattd stnictures. Probably they are in most cas&s true diabase-tuffs, bat 
sometimes they may be forms of diabase-lavas, which, like the stratified formations in 
which they lie, have undergone alteration, and in particular have acf^uired a more or 
less distinctly fissile structure, as the I'esult of lateral pressure and internal crushing.^ 

4. Fragmental Rocks of Organic Origin. 

This series includes ileyK»sits fonned either by the gi'owth and decay of oiganisnu 
in sitUf or by the transjKnt and subse*[uent accumulation of their I'emains. These msy 
be c(mveniently gi'ouped, according to their predominant chemical ingredient, into 
GalcareoiLs, Siliceous, r'hos])hatic, Carbonaceous, and Feniiginous. 

1. CALOAKKors. — Besiiles the calcareous formations which occur among the stratified 
fuystalline rocks as results of the dej^sition of chemical ju-ecipitates (p. 149), a more im- 
|K)rtant series is derive<l fi*om the remains of living organisms, either by growth on the 
s|>ot or by transport and accumulation as mwrhanical sediment. To by far the larger 
l»art of the limestones intercalated in the rocrky framework of our continents, an organic 
origin may with prol>ability be assigned. It is tme, as has been above mentioned (p. 122), 
that limestone, formed of the remains of animals or plants, is liable to an internal crysttl- 
line rearraugt-ment, the effect of which is to obliterate the organic stnicture. Hence in 
many of the older limestones, no trace of any fossils can be detected, and yet these rocks 
were almost certainly formed of organic remains. An attentive microscopic study of 
organic calcareous structures, and of the mode of their replacement by crystalline calcite, 
sometimes detects indications of former organisms, even in the midst of thoroughly 
crystalline materials.^ 

' Trans. Jioy. Sik\ Kilin. xxix. p. 514. 

" C. Koch, JahHK Ver. Xai. Xassaa, xiiL (1858), 216, 238. J. A. Phillips, Q, J, QtoL 
!ktr. xxxii. p. 155, xxxiv. p. 471. 

' Sorby, Address to O'eol. Society^ February, 1879, and the paper of Messrs. Comish 
and Kendall, cited untey p. 122. Giimbel lias suggested that the different durability of the 
calcitc and aragonite oigauic forms may be due rather to structure than mineral composition* 



PARTiigvii FRAGMENTAL ROCKS 139 

Limestone, composed of the remains of calcareous organisms, is found in layers 
which range from mere thin laminse up to massive beds, several feet or even yards in 
thickness. In some instances, such as that of the Carboniferous or Mountain limestone 
of Britain and Belgium, and that of the Coal-measures in Wyoming and Utah, it occurs 
in continuous superposed beds to a imited thickness of several thousand feet, and extends 
for hundreiis of square miles, forming a rock out of which picturesque gorges, hills, and 
table-lands have been excavated. 

Limestones of organic origin present every gradation of texture and structure, from 
mere 8oft calcareous mud or earth, evidently comjwsed of entire or cnnnbled organisms 
up to solid com|iact crystalline rock, in which indications of an organic source can 
hardly Iw perceived. Mr. Sorby, in the address already cited, called renewed attention 
to the im^wrtance of the form in which carbonate of lime is built up into animal 
stmctureH. Quoting the opinion of Rose expressed in 1858, that the diversity in the 
state of preservation of different shells might be due to the fact that some of them had 
their lime as calcite, others as aragonite, he showed that this opinion is amply supported 
by microscopic examination. Even in the shells of a recent raised beach, he observed 
that the inner aragonite layer of the common mussel ha<:l been completely removed, 
though the outer layer of calcite was well preserved. In some shelly limestones con- 
taining casts, the aragonite shells have alone disappeared, and where these still remain 
represented by a calcareous layer, this has no longer the original stnicture, but is more 
or less coarsely crystalline, being in fact a pseudomorph of calcite after aragonite, and 
qnite unlike contiguous calcite shells, which retain their original microsco[)ical and 
optical characters. ' 

The following list comprises some of the more distinctive and im^iortant forms of 
organically-derived limestones. 

Shell-Marl — a soft, white, earthy, or crumbling deposit, fonne<l in lakes and 
ponds by the accumulation of the remains of shells and Entomostraca on the bottom. 
When such calcareous dei)osits become solid compact stone they arc known as fresh- 
waiUr {lanistrine) limestones. These are generally of a smooth texture, and either 
dull white, ]iale grey, or cream-coloured, their fracture slightly conchoidal, rarely 
splintery. 

Lumachelle — a compact, dark grey or brouTi limestone, charged with ammonites 
or other fossil shells, which are sometimes iridescent, giving bright green, blue, orange, 
and dark red \mX» (fire-marble). 

Calcareous (Foraminiferal) Ooze — a white or grey calcareous mud, of organic 
origin, found covering vast areas of the floor of tin; Atlantic and other oceans, and 
formed mostly of the remains of Foraminifcra^ [wirticularly of forms of the genus 
Globigerina (Fig. 24). Further account of this and other organic deep-sea dejwsits is 
given in Book III. Part II. Section iii. 

Shell- Sand — a deiK>sit comjwsed in great measure or wholly of comminuted shells, 
found commonly on a low shelving coast exjwsed to prevalent on-shore winds. When 
thrown above the reach of the waves and often wetted by rain, or by trickling runnels of 
water, it is apt to become consolidated into a mass, owing to the solution and rede|)osit 
of lime round the grains of shell (p. 122). 

Coral- rock — a limestone formed by the continuous growth of coral-building ix)lyps. 
This substance affords an excellent illustration of the way in which organic stnictui-e 
may be eflhoed from a limestone entirely formed of the remains of once living animals. 
Though the skeletons of the reef-building corals remain distinct on the upper surface, 
those of their predecessors beneath them are gradually obliterated by the passage 
through them of percolating water, dissolving and redepositing calcium carbonate. We 
can thus understand how a mass of crystalline limestone may have been produced from 

* The student will find the address from which these citations are made full of suggestive 
matter in regard to the origin and subsequent history of limestones. 







ClMlk-a wLiV -.ft i.^.-k. iiL-a-TH i.. 
,:i].-:ir».r., t1<iir <l"[h-.'l :'r.>]ii t<» i^tiMiti' 
••tli-r iiuriii- fr'^'i]iL«iii>. Ily tiiakiii>; t)i{ 
ill" tll^.'^n■:>>|■•i, Sorliy Iia* r<-uii>l that ¥<.< 
•I>-Mi-L>'it •■ulU lit i-im\aitiKivr',v Aa\lon-\ 
■ ■{ rl.« ^..k l.j- liulk IV ±:\'ti,- ntiiaiii 
i.-iil'j»ni''ri- Idnr nf /^'nyivmin*. t'l^a^K-i 
-l-'iii,"-. *i-- It i- not ■)iiif Uk" aiiv ki 
ii.v.r-.rEiMti-ii •if viMlk frm,! tli.- uflJi^U^ 



.l-'l.t itl;.':'-.li^ll 

ii.i.- r..:l:- 
.i<Ul Ell' 



,1.1 t-U,. 



. ill til- till 

LiiuKi 



.'ur1>-iur-r< 



iiii'li. -liliiif: till' liii)9'r>. foniipd of a five 
'i.vim.'v.'r.V'i. ruliiniidi^rins, iiiolliiski, ud 
o •<! till- iiH'k >ttii rxaiiiiiuiig tliein nndfr 
ifiTa. i-artii'uUtly 'tl-Ai^riiui, and nugit 
r>-niis. ]>r<>l<i)>ly raiMtitutr less tliau hil[ 
■u-istiii!; cf •U-Ui-knl jmnna of the outer 
"J,--. I. /'.^■'.■-i. rvhiuiKli-miH, spii'uln ef 
IV kiiKivn ii|.-Wti c],><'[.-si-a •lq<•>^it. A tuicmwoidc 
;ii>-mil..u.| ut' I.il1r -linu'i^l thut. )«»ides tlie luoi] 
IIS iiiiiiiii.> uniiiii- JTul I'ly-luU •>r<|imrtz, tourmaliiK, 
III'"' uiiiii'L^il.-' l>'iii^' aniMii;; tlii> iii<wt videly diOnMil 
i-r .-•'■liiiii'iii-' tliiit uro ili'i'tvtil from the denudatka 

t-.>iii--ii rock i,-0Tii[><>M:-t ill .i,Teat jart of cryBtalliBt 
m. i-ruU ami iiio1luAkT<. it varira in i-olmir fron 
. iif liliii;.L-;,Ti-y M-uioIliiim ytflKni' or lirown, hm 
ivii 1<Wk iniUicr. It j- aliniiilaiit aiuoiig Palmuotc 
•■ r<|i>i.-ially ■.■Iiiii-au'trtUtif <if tlir lower jart of tUe 



' Se« Uitu'* •".'•ml ali'l I'l^ral Isbiid>.' i-. 3Sl : aUi tlw 
(.'arlMniremu^ timc-t-ui'' in tbe ]'ivMn; rultinur. Ihii^-mt I 
ipa-'ivt linivtaDv* of Bi^l^iiini haw Ivtii fonin^J i-y tvcf-lii 
allicil ori;iai<nii. 

■ U Uy*iiJi, .1"". .1*. ';,-.,/. .V,. ,/. ivii. uw,. ]'. -Iss. 



►ART n § vii FRAOMENTAL ROCKS 141 

2. SiLK'EOUs. — Silica is directly eliminated from both fresh and salt water by the 
rital growth of plants and animals. (Book III. Part II. Section iii.) 

Diatom-earth, Tripolite (Infusorial earth, Kieselgiihr) — a siliceous de[>osit forme<l 
;hiefly of the fnistule-s of diatoms, laid down both in salt and in fresh water. "Wide 
ireas of it are now being deposited on the bed of the South Pacitic {Diatom-ooze^ Fig. 
181). In Virginia, United States, an extensive tract occurs covered with diatom-earth 
to a depth of 40 feet. It likewise underlies i)cat-mosses, probably as an original lake- 
le|M)sit. It is used as Tripoli powder for polishing purposes (see p. 481). 

Radiol ailan ooie — a |)ale chalk-like abysmal marine deposit consisting mainly of 
the remains of siliceous radiolarians and diatoms. It is further referred to in Book III. 
Part II. Section iil ' U 

Flint (Silex, Fenerstein) — a grey or black, excessively com)>act rock, with the hard- 
ness of quartz and a perfect conchoidal fracture, its splinters being translucent on the 
edges. Consists of an intimate mixture of crystalline insoluble silica and of amorphous 
silica soluble in caustic potass. Its dark colour, which can be destroyed by heat, aiises 
chiefly from the presence of carbonaceous matter. Flint occurs principally as nodules, 
rli8|iersed in layers through the Upper Chalk of England and the north-west of Euroi)e. 
It fre«^uently encloses organisms such as si)onges, echini and brachiopods. It has l>een 
deposited from sea-water, at first through organic agency, and subsequently by direct 
chemical precipitation round the already dej)osited silica. (Book III. Part II. Sect, iii.) 
Chert (phtanite) is a name a]>plied to impure calcareous varieties of flint, in layers and 
nodules which are found among the Paleozoic and later fonnations, especially but not 
exclusively in limestonea^ In some cases, as in the spicules of s|K)nges, the silica has 
had a directly organic origin, having been secreted from sea-water by the living 
oi^gauisms ; in other cases, where for example we find a calcareous shell, or echinus, or 
coral, converted into silica, it would seem that the sul)stitution of silica for calcium- 
carbouate has been effected by a process of chemical pseudomorphism, either after or 
daring the formation of the limestone. The vertical ramifying masses of flint in Chalk 
show that the calcareous ooze had to some extent accumulated before the segregation 
of these masses.^ 

3. Phosphatic. — A few invertebi-ata contain phosphate of lime. Among these may 
be mentioned the brachiopods Lingula and Orbicula;^ also Conulariay SerpulitrSj and 
some recent and fossil cnistacea. The shell of the recent Lingula oralis was found by 
Hunt to contain, after calcination, 61 jMjr cent of fixed residue, which consisted of 85 '70 
per cent of phosphate of lime; 11*75 carbonate of lime, and 2*80 magnesia. The 
bones of vertebrate animals likewise contain about 60 ])er cent of phosi»hate of lime, 
while their excrement sometimes abounds in the same substance. Hence dei)osits rich 
in phosphate of lime have resulted from the accumulation of animal remains from 
Silurian times up to the present day. Associated with the Bala limestone, in the Lower 
Sihirian series of North Wales is a band comjiosed of concretions cemented in a black, 
graphitic, slightly phosphatic matrix, and containing usually 64 ]»er cent of phosphate 
of lime (phosphorite)."* The tests of the trilobites and other organisms among the 
Gambrian rocks of "Wales also contain phosphate of lime, sometimes to the extent of 20 
per cent.* Phosphatic, though certainly far inferior in extent and importance to cal- 

* Consult Hull and Hardman, Trans. Roy. DuUin Soc. i. (1878), p. 71. Renard, 
BuU. Acdd. Roy. Belgiqun, 2d ser. vol. xlvi. p. 471 ; Sollas, Ann. Mag. yat. Hist, vii. 
(1881), p. 141 ; Scientific Proc. Roy. iJiiNin Soc. vi. (1887), part i. G. J. Hinde, Geol. 
Mag. 1887, p. 435. Bands of radiolarian chert occupy persistent horizons among the 
Lower Silurian rocks of southern Scotland. 

* On formation of chalk-flints, see Book III. Part II. Section iii. § 3. 

' Sterry Hunt, Amtr. Journ. Soc. xvii. (1854), p. 236. Logan's 'Geology of Canada,' 
18^ p. 461. 

** D. C. Davies, Q. J. Geol, StK. xxxL p. 357. ^ Hicks, op. cit. p. 368. 



142 (iEoGSn^ Y book n 



earrctiLs, and even to siliceous, forniatioii.% are often of singular geological interest. The 
following examples may serve as illustrations.* 

Guano — a (IqK)sit consisting niainl}' of th«' droppings of sea-fowl, funned on isUndi 
in rainless tracts off tlie western coasts of South America and of Africa. It is a brown, 
light, }K>wdeiT substaui-c with a jieculiar animoniacal odour, and o(*cnrs in de|i08itt 
sometimes more than 100 feet thick. Analyses of American guano give — combustibk 
organic matter and acids, 1 1 *3 ; ammonia (carlN^nate, urate, kc, ), 31 '7 : fixed alkaline 
salts, sulphates, phosphates, clilori<ies, &c., 8*1 ; phosi»liates of lime and xiiagneaiay 
22 'f> ; oxalate of lime, 2*6 : sand and earthy matter, 1*6 ; water, 22*2. This remarkable 
.su1)stanc«' is highly valuable as a source of artificial manures. (Book III. Part IL 
Section iii.; 

Bono-Brocoia — a de^xtsit consisting largely of fragmentary' l>ones of living or 
extinct species of mammalia, found sometimes under stalagmite on the floors of lime- 
stone caverns, more or less uiixc<l with earth, san«l. or lime. In some older ^ct^^logicd 
formations, bone -beds occur, formed lai^ly of the remains of reptiles or fishes, as tbe 
"Lias Ijone-lx-*!," and the **Ludluw l>one-l»erl." 

Coprolitic nodules and beds - — ai-e foniied of the accumulateil excrement (coprolites) of 
vertebrated animals. Among the Carl>oiiiferou8 shales of the basin of the Firth of Forth, 
coprolitic nodules are abundant, together with the l>ones and scales of the lai^r ganoid 
fislics which voided them : abundance of broken scales and bones of the smaller ganoidi 
can usually l>e observed in the coprolites. Among the Lower Silurian rocks of Canada, 
numerous [ihosiiliatic nodules, su]i]N)sed to l)e of coprolitic origin, «K*cur.* The phos- 
phatic Ix'ds of the Cambridgeshire Cretaceous ro<.-ks are now largely worked as asouite 
of artificial manuiv. In iN>pular and e^Jlecially comnicix'ial usage, the word '* coprolitic'* 
is applietl to no<liilar deiMisits which can W woikivl for pho>phHte of lime, thongh tlwy 
may contain few or no true coprolites. 

Fhosphatic Chalk. — In the ('halk of France and lii'lgium, more s]iariugly in that of 
England, certain layei"s occur wheiv the original calcareous matter has been replaced to 
a considerable extent V>y phasphate of lime. Such 1>ands have freijuently a browniili 
tint, which on examination i> found t<i re^ult from the abundance of minute brown 
grains comiNuu'd mainly of [»h«i>pliatc. The foiaininifera ami other minuter or fragment- 
ar\* fossils have In-en changeil into this brown Hibstnnce. The ]>ro|M)rtion of phosphate 
of lime ranges up to 4.'! jht cent or more."* 

4. CAKiK>NAi*Kors. — Tlic formations here inclndc<l have almost always resulted from 
tbe decay and entombment of vegetation on the sjiot where it grew, sometimes by the 
drifting of the plants to a distance and their consolidation there. (See Uook III. Part IL 
Section iii. § 3.) In the latter case, they may be mingled with inorganic sediment, 
sfj as to pass into carbonaceous shale. 

"^esX — vegetable matter, more or less decomiKised and chemically altered, foond 
throughout tenqHirale climates in bogg}- places wlieiv mai-shy plants grow and deeiy. 
It varies from a pale \ellow <»r brown librous sul>stance, like turf or compmMd 
hav, in which the plant -ivmains are abundant and conspicuous, to a comi>act dark 
brown or black material, res»'mbling black clay when wet, an«l some varieties of lignitr 
when dried. The nature antl jiroiMirtions of the constituent elements of peat, afUr 
Iwing dried at 100* C, are illustrated by the analysis of an Irish example which gave— 
carbon, 60 48 ; hydrogen, 6 10 : oxygen, 32*;»5 ; nitrogen, 0*88 ; while the ash was 3*30. 

^ For an exhaustive account of deposits of pha-phate of lime, see R. A. F. Penroae Jr., 
Jli'll. l\S. th-ttl, Svrr. No. -Itj, 1888, also^^xWr", Book. III. Part II. Sect. iii. § 8. 

- On the origin of phosphatic nmhiles ami beds, see Gruner, Bull. Soc. Oiol. Franetf 
xxviii. (2nd ser.), p. 62. Martin, op. eit. iii. (3rd ser.), \\ 273. 

' Logan's * Geology of Caiia<l:i,' p. 461. 

* See A. F. Kenard and J. Cornet, BuU. Acaif. Ruy. BcOjique, xxi. (1891), p. 126. 
A. Strahan, Quart. Jouni. O'eoi. »St«-. xlvii. (1891). 



PART n § Tii FRAGMENTAL SOCKS 143 

There is ftlvsjs a Urge proportion of water wliioh cannot be driven olT even by 
diying the pe«t. In the maauiocture of comprensed peat for fuel thin constituent, 
which of couTM leasena the value of the ]>eat aa com]>arEd with an eijual weight of 
coal, is driven oiT la a great extent by chojiping the peat into line pieces, and thereby 
«X[wang a large surface to eva^ioration. The ash varioa in amount fi'om less than 
1-00 to more than S5 per cent, and consists of sand, clay, feme oxide, sulphuric acid, 
and minute proportions of lime, iK>da, potash and magnesia.' Under a jiresgure of 
MOO atmosphersB peat ia converted into a hard, black, brilliant substance having the 
phjakal aspect of coal, and allowing no trace of ovganic structure.^ 

Ugidt* (Biovn Coal) — compact or earthy, compressed and clieinjrally altered 
rcf^etable matter, often retaining a lamellar or ligneous texture, with stems sliotting 
woody fibn crossing each other in all direi^tioiiB, It varies from ]>ale brown or yellow 
to deep brown or black. Some shade of brown is the usual colour, nhence the name 
£nnm Ooat, by which it is often known. It contains from 5S to 75 per cent of carbon, 
baa a apsciflc gravity of 0-5 to I'S, bums easily to a light ash with a sooty flame 
and a strong burnt smell. It occurs in beds chiefly among the Tertiary strata, under 
conditions similar to those in which coal is found in older formations. It may 1>e 
nguded as a stage in the alteration and mineralization of vegetable matter, inter- 
mediate between peat and ti-ue coal. 

Coal — a compact, usually brittle, velvet-black to pitch-black, iron-black, or 
dull, sonietiiQes brownish rock, with a greyish. black or brown atreak, and in some 
Torietiea a distinctly cubical cleavage, in others a conchoidal fracture. It contains 
bam 75 to 90 per cent of carbon, and a small [lerccntage of sulphur, generally in 
the form of iron.disnlphide. It has a specific gravity of 1-2-1 'S.^, and bums n-itb com- 
paiatiTe readiness, giving a clear flame, a strong 
anxnatic or bituminous smell, some varieties fus- 
icig and caking into cinder, others burning away 
to a mere white or red ash. Though it consists 
of campresaed vegetatiou, no trace of □rgBiLLc 
structure is usually apparent.' An attentive 
examination, however, will often disclose poitions 
of stems, leaves, be. or at least of carbonized 
woody fibre. Some kinds are almost « holly made 
np of the spore. cases ot lyco]iO(liaceous plant.i 
fFig.25). Thete is reason to believe that different 
Tarietiea of coal may have arisen from original 
divetvties in the nature of the vegetation out of 
which they were formed. The accompanying 
table ahowa the chemical gradation between uii- tig. ij.—'ilicTiiscvjnv MtruitiiHollialki'ilh 
•Itarad vegsUtion and the more highly miner- C.-i,.ho-nii^ LyKpodiactou-Sponin- 
Ji^d for^ of coal. ^.(u,^..f,^m>V,«^Ur.X 

' Sm Senft's 'Humus-, Marscli-, Tori- nud Liuionlt.bililuugeu.' Leigizig, 1862. J. J. 
FtUh, ' Ueher Torf nod Dopplerit,' Ziiritb. 1883, and tlie various nieniolla quoted /losUa, 
p. 478. 

' ^Ting, Butt. Acad. Roy. BmieUta. ilix, (1880), p. 367. 

' On the iDJInence ot pressure on tlie formation of coal, see Frfmy, Com/tl. rend. 
aOth May 1876. Spring, Bull. Aead. Hoy. Bnixdlfs. 1880, p. 387. 




144 GEOGNOi^Y 






BOOKn 


Table showinu the <;HAi)rAL Chax<;] 


K IN Composition from Wood to Charcoai.' 


Substance. 


Carbon. H 

100 ; 

100 
100 

100 ' 

100 
100 
100 1 

1 


yilrt^n 


Oxygen. 


Dinponble Hydfo- 

gen, i.e., ovn sad 

above what i« »' 

quired to fonu water. 


1 1. AVooil (mean of several analyses . 

2. Peat ( „ „ „ ) . 

3. Lignite (mean of 15 varieties) . . 

4. Ten-yarfl coal of S. Stalfonlshire \ 
1 basin j 

1 r». Steam coal from the Tyne . . . 

1 6. Pentrefelin coal of S. Wales . . 

7. Anthracite from Pennsylvania, U.S. 


12-18 
9-85 
8-37 

612 

5-91 
4.75 

2.84 


83.07 
65.67 
42.42 

21. 23 

18.32 
5.28 
174 


1-80 
2-89 
8.07 

8*47 

8-62 
4-09 
2-63 



Coal o<'curs in seams or 1)C(ls intercalated l>etween strata of sandstone, shale, fireclay, 
&c., in geological formations of Palieozoic, Secondaiy, and Tertiary age. It should In 
rcmemhei*ed that the word coal is rather a i>o])ular than a scientific term, beiag 
indiscriminately applied to any dense, black mineral substance capable of beu^ 
used as fuel. Strictly cmi)loyed, it ought only to lie used with reference to beds «f 
fossilized vegetation, the result either of the growth of i)lants on tlie spot or of the 
drifting of them thither. 

The following analyses show the chemical composition of peat, lignite, and some of 
the i)rinci[)al varieties of coal - : — 



real. 

Devon- 
nhiiv. 



Lignitf. 

Bovry 
Tracj'y, 
Dcvdu. 



Conl. 

Xorthuni- 
txrland. 



Non-Cak- 
ing Cotil. 

S.«taflronl. 
shire. 



Carbon .... 

Hydrogen . 

Oxygen 

Niti"ogen 

Sulphur 

Ash .... 

SiMJcitic gravity 



54.02 
5-21 

28-18 
2-30 
0..'')6 
9.73 



0-850 



66-31 
5-63 

22-86 
0-57 
2-36 
2-27 



78-69 
6-00 

10-07 
2-37 
1-51 
136 



1-129 1-259 



CannH 
Coal, 

Wigan. 



78-57 
5-29 

12-88 
1-84 
0-39 
1-03 



1-278 



80-07 
5.53 
8-08 
2.12 
1-50 
270 



1276 



Anlkn- 
file, 

S.Wal6i. 



90-S9 
3-28 
2-98 
0-8S 
0*91 
]-61 



1893 



These analyses are exclusive of water, which in the i»eat amounted to 25*56, and ii 
the lignite to 34-66 \^r cent. 

Anthracite — the most highly mineralized form of vegetation — is an iron-blaok to 
velvet -black substance, with a strong metalloidal to vitreous lustre, hard and biittK 
containing over 90 per cent of carbon, with a si)ecitic gravity of l-SS-l*?. It kindkl 
with difficulty, and in a strong draught burns without fusing, smoking, or smelling; bnt 
giving out a great heat. It is a coal from which the bituminous parts have beM 
eliminated. It occurs in beds like ordinary coal, but in positions where probaUy tt 
has been subjected to some change whereby its volatile constituents have been ezpelkdi 
It is found largely in South Wales, and sparingly in the Scottish coal-fields where the 
oixlinary coal-seams have been approached by intnisive masses of igneous rock. It it 
largely dcvelo])e<l in the great coal-field of Pennsylvania. Some Lower Silurian ahaki 



1 Percy's * Metallurgy,' vol. i. p. 268. 



' From Percy's * Metalluigy,' voL L 



PAHT II § vii FRAGMENTAL ROCKS 145 



are black from difiused anthracite, and have in consequence led to fruitless searches for 
coal. 

Oil-shale {Brandxhiefer) — shale containing such a proportion of hydrocarbons as 
to be cajMible of yielding mineral oil on slow distillation. This substance occurs as 
ordinary shales do, in layers or beds, intei-stratified with other aqueous deposits, as in 
the Scottish coal-fields. It is in a geological sense true shale, and owes its |)eculiarity 
to the quantity of vegetable (or animal) matter which has been preserved among its 
inorganic constituents. It consists of tissile argillaceous layers, highly impregnated 
with bituminous matter, |)assing on one side into common shale, on the other into 
cannel or ^Mirrot coal. The richer varieties yield from 30 to 40 gallons of cnide oil to 
the ton of shale. They may be distinguished from non -bituminous or feebly bituminous 
shales (throughout the shale districts of Scotland), by the j>eculiarity that a thin paring 
curbt up in front of the knife, and shows a brown lustrous streak. Some of the oil- 
shales in the Lothians are crowded with the valves of ostracod crustaceans, besides 
scales, coprolites, &c., of ganoid fishes. It is iK>ssible that the bituminous matter may 
in some cases have resulted from animal organisms, though the abundance of plant 
remains indicates that it is probably in most cases of vegetable origin. Under the 
name **pyro8chists" Sterry Hunt classed the clays or shales (of all geological ages) 
which are hydrocarbonaceous, and yield by distillation volatile hydrocarbons, in- 
flammable gas, kc. 

Petrolenm, a general term, under which Ls included a series of natural mineral 
oils. These are Huid hydrocarbon com[>ounds, varying from a thin, colourless, watery 
liquidity to a black, opaque, tar-like viscidity, and in si)ecific gravity from 0*8 to 1*1. 
The paler, more limpid varieties are generally called naphtha, the darker, more 
viscid kinds mineral tar, Avhile the name petroleum, or rock-oil, has been more 
generally applied to the intermediate kinds. Petroleum occurs s[>aringly in £uroi>c. 
A few localities for it are known in Britain. It is found in large (quantity along the 
country stretching from the Car{)athians, through Gallicia and Moldavia, also at 
Baku on the Caspian.* The most remarkable and abundant dis]>lay of the substance, 
however, is in the so-called oil-regions of North America, i>articularly in Western Canada 
and Northern Pennsylvania, where vast ([uantities of it have been obtained in recent 
years. In Pennsylvania it is found especially in certain porous beds of sandstone or 
"sand-rocks," which occur as low down as the Old Re<l Sandstone, or even as the top 
of the Silurian system. In Canada it is largely present in still lower strata. Its 
origin in these ancient formations, where it cannot be satisfactorily connected with any 
destructive distillation of coal, is still an unsolved problem. 

Aiphalt — a smooth, brittle, pitch -like, black or brownish -black mineral, having a 
resinous lustre and conchoidal fracture, streak i>aler than surface of fracture, and 
specific gravity of 1 "0 to 1 '68. It melts at al)out the temperature of boiling water, 
and can be easily kindled, burning with a bituminous odour and a bright but smoky 
flame. It is comi>08ed chiefly of hydrocarbons, with a vai-iable admixture of oxygen and 
nitrogen. It occurs sometimes in association with petroleum, of which it may be 
considered a hardened oxidized form, sometimes as an impregnation filling the pores or 
chinks of rocks, sometimes in indei>endeut beds. In Britain it api>ears as a product of 
the destructive distillation of coals and carbonaceous shales by intrusive igneous rocks, 
as at Binny Quarry, Linlithgowshire, but also in a number of places where its origin is 
not evident, as in the Cornish and Derbyshire mining districts, and among the dark 
flagstones of Caithness and Orkney, which are laden with fossil fishes. At Seyssel 
(Departement de I'Ain) it forms a de]>osit 2500 feet long and 800 feet broad, which 
yields 1500 tons annually. It exudes in a liquid form from the groimd round the 

* AlHch, Jahrb. OeoL Reichmmst. xxix. (1879), p. 165. Trautschold, ZeiUch. DeutscJi. 
GeoL Qtt. xzvi. (1874), p. 257. See poateay Book III. Part I. Sect. i. § 2 where other 
aathorlUes are cited, y^ 



1 46 GEOGNO^ Y book u 

Iwrders of the Dead Sea. In Triuidad it fonns a lake IJ mile iii circumference, which 
is cool and solid near the shore, but increases in teini)eraturc and softness towards the 
centre. 

Graphite. — This mineral occui*s in masses of sufficient size and ini|x)rtance to deserre 
a i)lac>e in the enumeration of carbonaceous rocks. \Xs mineralogical characters hare 
already (p. 67) been given. It oix'ui-s in distinct lenticular lieds^ and also diffused in 
minute scales, through slates, schists, and limestones of the older geological formatioui^ 
as in Ciunberland, Scotland, Canada, and lk>hemia. It is likewise found occasioually 
as the result of the alteration of a coal seam by intiiisive Itasalt, as at New Cumnock iu 
Ayrshire. 

5. Fekruoinous. — The decomposition of vegetable matter in marshy places and 
shallow lakes gives rise to cei-tain organic acids, which, together with the carbonic 
acid so generally also present, decom|K)se the ferniginous minerals of rocks and cany 
away soluble salts of iron. Ex^tosure to the air leads to the rax)id decom|K>sitiou and 
oxidation of those solutions, which consequently give rise to precipitates, consisting 
iwiitly of insoluble basic salts and jwrtly of the hydrated fenic oxide. Tliese prci^ipitatei, 
mingled with clay, sand, or other mechanical impurity, and also with dead and decay- 
ing organisms, fonn dejwsits of iron-ore. OiK?rations of this kind ap|iear to liave 
been in progress from a remote geological antiquity. Henw ironstones with traces 
of associated organitr remains l>elong to many ditlerent geological formations, and an 
being formed still.* 

Bog Iron-Ore (Lake-ore, minerai des marais, Sumpferz) — a dark-brown to black, 
earthy, but sometimes compact mixtui*e of hydmted |»eroxide of iron, phosphate of iron, 
and hydrated oxide of mangamise, fre(juently with clay, sand, and oi'ganic matter. An 
ordinary specimen yielded, ]ieroxide of iron, 6'J'59 ; oxide of manganese, 8*52; saud, 
11 '37; phosphoric acid, 1*50; sulphuric acid, traces; water and organic matter. 
16*02=100*00. liog iron -ore may either be fonned in situ from still water, or may be 
laid down by currents in lakes. Of the former mode of foi-mation, a familiar illustration 
is furnished by the " moor-band i>an " or hard ferniginous crust, which in boggy placet 
and on some ill-drained land, fonns at the bottom of the soil, on the top of a stiff and 
tolerably imi)ervious subsoil. Abun«iant bog-iron or lake-ore is obtained from the bottoms 
of s<mie lakes in Norway and Swedcni. It forms everywhere on the shallower slopes 
near banks of reeds, where there is no strong cunent of water, occiu'ring in granular 
concretions (Bohnerz) that vary from the size of grains of coarse gunpowder up to 
nodules 6 inches in diameter, and forming layers 10 to 200 yards long, 5 to 15 yards 
broad, and 8 to 30 inches thick. These deposits are worked during winter by inserting- 
l)erforated iron shovels through holes cut in the ice ; and so rapidly do they acc^umulate, 
that instances are known where, after having l)eeu completely removed, the ore at the 
end of twenty-six yeai-s was found to have gathered again to a thickness of several 
inehew. A layer of l(X>se earthy ochre 10 feet thick is believed to have formed in 600 
years on the floor of the Lake Ti.sken near the old coj>]»er mine of Falun in Sweden.* 
Accoi-ding to Ehrenl^erg, the fonnation of bog-ore is due, not merely to the chemictl 
actions arising from the decay of organic matter, but to a power ]M)ssessed by diatoms of 
sei»arating iron from Avater and depositing it as hydrous iKToxi<le within their siliceous 
framework. 

AluminouB Yellow Iron-Ore is closely ivlatcd to the foregoing. It is a mixture 
of yellow or pale brown, hydrated peroxide of iron, with clay and sand, sometimes 
with silicate of iron, hydrated oxide of manganese, and carbonate of lime, and oeeurB 
in dull, usually i)ulvenilent grains and nodules. Occasionally these nodules nuiy 
\ye observed to consist of a shell of harder material, within which the yellow oxide 

* See Senft's work already (p. 143) cited, p. 168 ; also jMtstea, Book III. Part II. 
Sect. iii. 

- A. F. Thoreld, Oevi, Fiireu. FOrfuinif, siiockholm, iii. p. 20, pontea, pp. 407, 488. 



FRAGMENTAL ROCKS 



147 



become* programTslj softer tawuds the centre, wliich U goraetlmea quite empty. Such 

concretiont Me known as letites or eagle-atones. Tliis ore occurs in the Coal-measurcH 

of 3«xony aiid Sileeia, also in the Harz, Baden, Bavaria, &c., and among the Jurassic 

rocks in England. 

Cakf-lrvaatoiu (Sphi^roeiderite) has been already (p. 78) Inferred to. It 

oconra abundantly tu nodules and beds in the Carboniferous system in most [arts of 

Europe. Tlie nodales are generally oval and flattened in foni 

Tsrying in size from a small bean up to concretions a foot c 

more in diameter, and with an internal system of radiating 

cracks, often filled with calcite (Fig. 26). la many cases, they / 7^ 

contain in the centre some organic substance, such as a copro- ' ~^ -^ 

lite, fern, oone, shell, or fish, that has served as a surface round 

which the iron in the water and the anrroimding mud could 

be precipitated. Seams of clay. ironstone vary in ttiickneHs 
from mere paper -like partings up to beds several feet deep. 
The Cleveluid seam in the middle Lias of Yorkshire is about 
SO feet thick. In the Carboniferous system of Scotland certain seams known as BI/kI:- 
band contain from 10 to 52 per cent of coaly matter, and admit of being calcined with 
the addition of little or no fuel. They are sometimes crowded with organic remains, 
eapecially lamellibranchs {Anthracotia. Anlhraeomya, kc.) and flsbes {Rhizodus, Mega- 
lidit/iyt, *c.) 

A microscopic examination of some block.bsnd ironstones reveals a very perfect 
oolitic structure, showing that the iron has either replaced an original calcareous 
oolit« or has been precipitated in water having such a gentle movement as to keep the 
graonles quietly rolling along, while their successive concentric layers of carbonate were 
being deposited. Hr. Sorby has observed in the Cleveland ironstones an abnormal form 
of oolitic structure, and remarks that one specimen bore evidence that the iron, mostly 







in the form of small crystals of the carbonate, had been introduced snbsequeutly 
fbntuition of the rock, as it had replaced some of the aragoiiito of the enclosed sliells. 
The subjoined analyses show the comjiosition of some varictieii of clay. 



the 







CIsy l™....r. 


Ulack Baii.[ 


Clev,L.i„l „i* 






(C«l in«.siir^ 




(Lii>), 








Sl'MllllKl. 


X-HltMTt. 


Peroxide of iron 




1-45 . 


. 2-72 . 


2.86 


Protoxide of iro 






. 40-77 . 


43-02 


Protoxide of manganese 


1-38 . 




0-40 


Alumina . 




6.74 . 




687 


Lime 




2.70 . 


. 0.90 . 


G-I4 (zini') 


Magnesia , 




217 . 


0.7-J , 


5-21 


FoUsh . 




0.6S . 






Silica 




17-37 . 


'. 10.10 '. 




Carbonic acid 




26.57 . 


. 26-41 . 


25-50 


Phosphoric «id 
Sulpfinric acid 




0.34 . 




1-81 










Iron pyrites 
Water 
Organic matter 




010 . 


'. 10 '. 


— 




2-tO '. 


01.-. 






9S-78 


100-00 


100-til 


Penxntage of iron . 




29.12 . 


. 34-80 . 


35-46 


' Address to Geol. Soc 


Februar)- 187B 








» See Percy's ' Metatu 


■gy,' vol. ii. B 


■chof, 'Chem 


und Ph.v». G« 


1.- .npp. (18711. 



1 48 (iKOOXOS Y BOOK II 



B. CkYSTALLINK, INrLl'l)IN«i Rocks KURMKI) KKOM CiIF.MKAL rilECIPITATIOX. 

This division consists mainly of chemical de|X)sits, but includes also 
some which, originally formed of organic calcareous debris, have acquired 
a crystiillinc structure. The rocks included in it occur as laminse and 
beds, usually intercalated among clastic formations, such as sandstone 
and shale. Sometimes they attain a thickness of many thousand feet, 
with hardly any interstratification of mechanically derived sediment 
They are being formed abundantly at the present time by mineral springs 
and on the floor of inland seas : while on the bottom of lakes and of the 
main ocean, calcareous organic accumulations are in progress, which will 
doubtless eventually acquire a thoroughly crystalline structure like that 
of many limestones. 

Ice. — So large an area of tlio earth's surface is covenKl with ice, that this su1>- 
stance deserves notice among geological formations. Ice is commonly and conveniently 
classitied in tAvo divisions, snow-ice and water-ice, accoitluig as it results from the 
comi»ressi(ni ami alternate melting and freezing of fallen snow, or from the freezing of 
the surface or bottom of slutets of water. 

Snow-ice (see Book III. Part II. Sect. ii. § 5) Is of two kinds. Ist, Fallen snow 
(m mountain sloiws above the snow-line gradually assumes a granular structure. The 
little crystalline needles and stai-s of ice are melted and frozen into rounded grannies 
which form a more or less com{)act mass known in Switzerland as Aev^ or Fini^ 2nd, 
When the granular neve slowly slides down into the valleys, it iicquires a more compact 
i'lystallinc stnicture an<l becomes glacier- in: Acconling to the researches of F. Elocke, 
glacier-ico is, throughout its mass, an irregular aggi'egatc of distinct ciystalline grains^ 
the boundaries of which form the minute capillary fissuivs so often described.* It* 
fitiiKttui'e thus closely corres|M>u«ls to that of marl>lo (p. ir»l). Olacier-iee in small 
fragments is white or colourless, and often shows innumerable tine bubbles of air, 
.sometimes also line jxirtieles of mud. In larger masses, it has a blue or green-blue tint, 
and displays a veined structure, consisting of parallel v(>rtical veinings of wlute ice 
full of air-bubbles, and of blue clear iee without air-bubbles. Suow-ice is formed 
above the snow-line, but may descend in glaciers far below it. It covei-s large areai 
of the more lofty mountains of the globe, even in tropical regions. Towards the 
]^oh^s it descends to the sea, where large pieces break olf and lloat away as icebergs. 

Water-ice (see Hook III. Tart II. .Sect. ii. § .'*) is formed, 1st, by the freezing of 
the surface of fresh water (river-ice, lake-ice), or of the .sea (Jce-foot, floe-ice, |iaek-ice); 
this is a compaet, clear, white (»r greenish ice. 2nd, by the freezing of the layer of water 
lying on the bottom of rivei*s, or the sea (bottom-ice, ground-ice, anchor-ice) ; this 
variety is more s[H)ngy, and often encloses mud, .sand and stones. * 

Rock-Salt (Sel gemmc. Steinsalz. ]>. 79) occurs in layers or lieds from less than an inch 
to many hundred feet in thickness. The .sidt dejwsits at Stassfiut, for example, are 1197 
feet thi<'.k, of which the lowest beds comprise 685 feet of pure rock-salt, with thin layers 
of anhydrite :J-inch thick dividing the salt at intervals of from one to eight iucLcs. Still 
more massive are the accumulations of Si>erenl>erg near Berlin, which have been bored 
through to a depth of 4200 feet, and tho.se of Wieliczka in Gallicia which are here and 
there more than 4G00 feet thick. 

^ Xt'HCs Juhrh, 18S1 (i.), p. 23. Grad and Duprr {Ann. Cluh, Alp. Franc. 1874) show 
how the characteristic structure of glacier-ice may lie revealed by allowing coloured solutions 
to ]iernieat« it. 

' On the properties of ice with some interesting geological bearings, see 0. PetterssoD, 
* Vega-Expeditionens Veteuskaiiliga lakttagelser,' vol. ii. p. 249, Stockholm, 1883. 



PARTn§vii STRATIFIED CRYSTALLINE ROCKS 149 



The more insoluble salts (notably gypsum or anhydrite) are apt to appear in the 
lower parts of a saliferous series. When purest, rock-salt is clear and colourless, but 
usually is coloured red (i)eroxide of iron), sometimes green, or blue (chloride or silicate 
of copper). It varies in structure, being sometimes beautifully crystalline and giving a 
cubical cleavage ; laminated, gi'anular, or less frequently fibrous. It usually contains 
some admixture of clay, sand, anhydrite, bitumen, &c. , and is often mixed with 
chlorides of magnesium, calcium, &c. In some places it is full of vesicles (not 
infrequently of cubic form) containing saline water ; or it abounds with minute cavities 
filled with hydrogen, nitrogen, carbon-dioxide, or with some hydrocarbon gas. 
Occasionally remains of minute forms of vegetable and animal life, bituminous wood, 
corals, shells, crustaceans, and fish teeth are met with in it. Owing to its ready solu- 
bility, it is not found at the surface in moist climates. It has been formed by the 
evaporation of very saline water in enclosed basins — a process going on now in many 
salt'lakes (Great Salt Lake of Utah, Dead Sea), and on the surface of some deserts 
( Kirgis Steppe). In different parts of the world, deposits of salt have probably always 
been in progress from very early geological times. Saliferous formations of Tertiary and 
Secondary age are abundant in Europe, while in America they occur even in rocks as old 
as the Upper Silurian period, and among the Punjab Hills in still more ancient strata.* 

CSamaUite — a chloride of potassium and magnesium (p. 79). It occurs in a bed 20 to 30 
metres thick which overlies the rock-salt in the saliferous series of Stassfurt, and has been 
found in other old salt deposits, as well as among the "salterns" or "salines " along 
the Mediterranean coast where the water of that inland sea is evaporated in the manu- 
fiw^ture of salt. It so closely resembles rock-salt that it was formerly included with it, 
bnt it is much less frequently met with. It is a valuable source for the manufacture of 
potash -salts. 

Lbnestone (Calcaire, Kalkstein), — essentially a mass of calcium -carbonate, some- 
times nearly pure, and entirely or almost entirely soluble in hydrochloric acid, some- 
times loaded with sand, clay, or other intermixture. Few rocks vary more in texture and 
composition. It may be a hard, close-gi*ained mass, breaking with a splinterj' or coi^- 
choidal fracture ; or a crystalline rock built uj) of fine ciystalline grains of calcite, and 
resembling loaf-sugar in colour and texture ; or a dull earthy friable chalk-like deposit : 
or a compact, massive, finely-granular roc*k resembling a close-grained sandstone or free- 
stone. As its hardness is about 3, it can easily be scratched with a knife and 
the white powder gives a copious effervescence with acid. The sjiecific gravity naturally 
varies according to the impurity of the rock, ranging from 2-r) to 2-8. The colours, too, 
vary extensively, the most common being shades of blue-grey and cream-colour passing 
into white. Some limestones are highly siliceous, the calcareous matter having been 
accompanied with silica in the act of deposition ; others are argillaceous, sandy, ferni- 
ginoos, dolomitic, or bituminous. By far the larger number of limestones are of organic 
origin ; though owing to internal re-arrangement, their original clastic character has 
frequently been changed into a crystalline one. Under the present subdivision are 
placed all those limestones which have had a distinctly chemical origin, and also those 
which though doubtless, in many cases, originally fonned of organic debris, have lost 
their fragmental, and have assumed instead a crystalline structure. (For the organic 
limestones see p. 139.) 

Compact, common limestone, — a fine-grained c-r}'8talline-granular aggregate. 
oecnrring in beds or laminie interstratified with other a«iueous de^wsits. When purest it 
is readily soluble in acid with effervescence, leaving little or no residue. Many varieties 
oocnr, to some of which separate names are given. HydrnuHc limestone contains 10 i>er 
cent or more of silica (and usually alumina) and, when burnt and subsequently mixed 
with water, forms a cement or mortar, which has the i^roperty of "setting" or hard- 

^ On salt deposits of various ages see A. C. Ramsay. Brit. Assin\ Rep. 1880, p. 10 ; 
also Index, futb voc. ** Salt Depasits. " 



1 5 GEOGNOH Y book ii 



euiiig under water. Limestones containing i>erhai>8 as nnieh as 25 \ter cent of silio, 
luniina, iron, &c., that in themselves would be unsuitable for many of the ordiniiy 
jiurposes for which limestones are used, can be emi>loyed for making hydraulic mortar. 
The^e limestones occur in beds like those in the Lia« of Lyme Regis, or in noduleg like 
those of Shem^ey, from which Roman cement is made. CctMntatonc is the name given to 
many pale dull feiTUginous limestones, which ctmtain an admixture of clay, and some of 
which can be profitably used for making hydmulic mortar or cement. Fetid limesixmc 
[stUikstein, sxcinestonc) gives off a fetid smell (sulphuretted hydrogen gas), when struck 
with a hammer. In some cases, the rock seems to have been do|K>sited by volcuiu 
springs containing decomi)08able sulphides as well as lime. In other instanoes, the 
odour may be connected with the decomposition of imbedded organic matter. In some 
i[uames in the Carboniferous Limestone of Ireland, as mentioned by Jukes, the freahly- 
broken rock may be smelt at a distance of a hundred yaixls when the men are at v«rk, 
and occasionally the sUuich beeomtis so strong that the workmen are sickened by it» and 
require to leave off work for a time. Canisfone is an arenaceous or siliceoOB li m e rt o wi 
|»artieularly characteristic of some of the Pala'ozoic red sandstone formations. JfolCm- 
ittonc is a decomiwsed siliceous limestone from wliieh most or all of the lime has 
been removed, leaving a siliceous skeleton of the rock. A similar decompoeiti<ni takfli 
]»lace in some fcnuginoiLs limestones, with the result of leaving a yellow skeleton of 
ochre. Common limestone, having been deposited in water usually containing other 
substances in sus])cnsion or solution, is almost always mixed with im]niTities, and 
where the mixture is sufficiently distinct it receives a si^'cial name, such as siliceooi 
limestone, sandy limestone, argillaceous limestone, bituminous limestone, dolomitic 
limestone. 

TraviTtine (calcareous tufa, rale -sinter) is the ]x>rous material de{>osited by cal- 
careous springs, usually white or yellowish, varying in texture from a soft chalk-like or 
marly substiince to a conijwict building-stone. (See Book III. Part II. Sect iii. §§ 3, 6.) 
StalfU'litc is the name given to the calcareous jiendant deposit formed on the roofs of 
limestone - caverns, vaults, bridge^s, &e. ; while the water, from which the hanging 
lime-icicles are derived, drii)S to the floor, and on further cvajjoration there, gives riie 
to the crust-like deix)sit known as Htahufmltt'. Mr. Sorby has shown that in the 
calcareous depasits from fresh water there is a constant tendency towards the produo- 
tion of calcite crystals with the ]»rinei}>al axis i)eriH*ndicular to the surface of deposit. 
Where that surface is curved, there is a radiation or eonvergenc^ of the fibre -like 
crystals, well seen in sections of stalactites and of some calcareous tufas (Fig. 108). 

Oolite, — a limestone formed wholly or in ]»art of more or less iH>rfectly spherical 
grains, and having somewhat the iwi)ect of fish -roe. Eacrh grain consists of successiTe 
I'oncentric shells of carbonate of lime, frequently with an internal radiating fibrous struc- 
ture, which gives a black cross between crossed Nicols (Fig. 27). The calcareous materud 
was <h;pusited round some minute iwrtide of .sand or other foreign body which was kept 
in motion, so that all sides could in turn become encrusted. Oolitic grains of this 
charact^T are now forming in the springs of Carlsbad (Sprudclstein) ; but they may no 
doubt also be [a-odueed where gentle currents in lakes, or in partially enclosed areas of 
the sea, keep grains of sand or fragments of shells drifting along in water, which is so 
charged with lime as to be nyidy to de|X)sit it xxyyow any suitable surfai^e. An oolitic lime- 
stone may contain much impurity. Where the calcareous granules are cemented in a 
somewhat argillaceous matrix the rwk is known in (lermany as Rogenstein. Where the 
individual grains of an oolitic limestone are as large as peas, the rock is called a piso- 
1 i te (i»ea-grit). The granules sometimes consist of aragtmite. Oolitic structure is found 
in limest(mes of all ages from Paheozoic down to recent times.* Mr. E. Wethered has 
recently |»ointed out that many oolitic grains show curious vermifonn twistings in their 

* Oolitic structure is fouinl even among the limestones of the Dalradian metamorphie 
series of Scotland (Islay) which may iwssibly be pre-Palieozoic. 



ii§vii STRATIFIED CBYSTiLLISE ROCKS l&l 

concentrio co»ta, which he regards aa of organic ongin either | laiit or animal 
tiella).' Id some instances oolites ha>e had their calcareous matter replaced 
bonate or oxide of iron, so as to become ootitic irotistoiies 

irblo {granular liraestoue) — a crystalline grauuUr aggregate composed of 
Uiw oalciU-grauuleii of remaikably uniform sue «ach of which las its own 
mdont twin lamella (often giving interference colours) and oleaiage lines This 
terUtic stmeture is well displajed when a thin slice of ordinary statuary marble 




•d nnJer the mieroBcojie (Fig. 28). Tyjiical marble is while, but tbe rock is also 
. grey, I'luE. green, red, hlack. or streaked and mottled, as may be seen iu the 
ana kinds ilsed for ornamental purposes. Its granular structure gives it a resem- 
to loaf-augar, whence the term " saccharoid " a]>plied to it. Fine silvery scales of 
r talc may ofleii be noticed ei-eu in the purest marble (Cipoli«o). Some crystalline 
ones aasociated with gneiss and schist are peculiarly rich in minerals,— mica, 
, tremolite, actinoHte, anthophyllite, loisite, vesuviauite, pyroxenes, and many other 

occurring there often in great abundance. These iuvhisions can be isolated by 
iug the surrounding rock in add (niile, p. 87). 

rble ia regarded by most geologists an a nietamorphic rock, that is, otic in which 
cium-carbonate, whether derived from an organic or inorganic sourve, has been 
y pecrystalliied i» situ. In the course of this change the oripnal clay, sand or 
mpuritics of the rock have been also crystaUized, ami now apjiear as the crystsllins 
IS just referred to. Harble occurs in beds and large lenticular masses associated 
^Btalline schists on many different Kcological horizons. In Canada it occurs of 
itian ; in Scotland of Cambrian ; in Utah of Up|>er Carboniferous ; in Southern 
I of Triassic, Jurassic and Cretaceous age. 

ondto (Magneaian Limestone) consists lypically of a yellow or white, crystalline, 
e aggregate of the mineral dolomite ; but the relative proportions of tbe calcium 
ignesium.carbonates vary indefinitely, so that every gradation can t>e found, from 
maitone without magnesium -carbonate up to jnire dolomite containing 4S-S5 ]>er 

that carl>onate. Ferrous carbonate is also of eomnion occurrence in this rock. ~ 
itore of dulomit« is usually distinctly crystalline, the individual crystals being 
nally so loosely helil together that the rock readily cnimbles into a ciystalline 

toL ilng. 1889, p. 196 ; Qunrl. J-mra. <i-:.,l. »x. \lii. (1890), p. 270. Mr. C. Reid 
gested that these tabular UMefi may )>e due to the <leposit of lime rouml organic 
t* {Al9^) like the calcareous incrustution formeii rouml fibres of hemp iu kettles and 



1 5 2 GKOGNOH Y book ii 

sand. A fissured cavernous structure? ai)]>arcntly due to a process of contraction duriiig 
the process of ^^ dot omit izatiou ," is of common occurrence : even in com|)act varieties, 
cellular sjwices occur, linerl with crystallized dolomite (Rauchwacke), the cryaftals of 
which are ofttm hollow and sometimes enclose a kernel of calcite. Other varieties are 
built up of spherical, botryoidal and iiTegularly-shaiKjfl concretionary roasses. Dolo- 
mite, in its more typical forms, is distinguishable from limestone by its greater hardneu 
(3'5-4«r)) higher sjKJcitic gra\ity (2-8-2'95), and much less easy solubility in acid. It 
occurs sometimes in beds of original de|>osit, associated with gj-psum, rock-salt and 
other results of the evaporation of satumted saline watei*8 ; it is also found replacing 
what was once ordinary limestone. The process by which carl)onate of lime it 
replaced by carlwnate of magnesia, is referred to in ]k>ok III. Part I. Sect. iv. § 2.* 
Dolomite sometimes forms pictuivsque mountain masses, as in the Dolomite Moiintaini 
of the Eastern Alj»s. 

Oypsum — a line granular to com|)act. sometimes fibrous or sparry aggregate of the 
mineral gj-psum, having a hardness of only 1 -5-2 (therefore scratched with the nail), and a 
specitic gi'avity of about 2-32, and being unaffected by acids ; hence readily distinguishable 
from limestone, which it occasionally resembles. It is normally white, but may be 
coloured gi'ey or brown by an admixture of clay or bitumen, or yellow and red by being 
stained with iron-oxi«le. It occurs in beds, lenticular intercalations and strings, usually 
associated with berls of red clay, ro<.'k-salt, or anhyrlrite, in formations of many 
various geological ix'rir»ds from Siluiian (Xew York) down to recent times. The 
Triassic gypsum deposits of Thuringia, Hanover and the Harz have long l)een lamoua 
One of them runs along the south flank of the Harz Mountains as a great l)and six niilei 
long and reaching a height of sometimes 430 feet. 

Gypsum furnishes a good illustration of the many different ways in which some 
mineral substances can originate. Thus it may be i>ro<iuced, Ist, as a chemical 
precipitate from solution in water, as when sea-water is evajjorated ; 2nd, through the 
decom]»osition of sulphides and the action of the resultant sulphuric acid u])on lime- 
stone ; 3nl, through the mutual deconii>ositinn of carbonate of lime and sulphates of iron, 
(ropper, magnesia, &c. ; 4tli, through the hydration of anhydrite ; 5th, thmugh the 
action of the suljiliuroiis vapoui-s and solutions of volcanic orifices ujKjn limestone and 
calcareous rocks.- It is in the tii-st of these ways that the thick beds of gyi^um assod- 
ated with rwk-salt in many geological formations have been formed. The first mineral 
to api»ear in the evajjoration of sea-water being gypsum, it has lieen precipitated on the 
floors of inland seas and saline lakes iK'fore the more soluble salts. 

Anhydrite, — the anhydrous variety of calcium -sulphate, occurs as a compact or 
granular, white, grey, bluish or reddish aggiegate in saliferous dejiosits. It is leas 
freijuent than gypsum, from which it is distinguished by its nmch greater hardness 
(3-3-ri;i and into which it readily i»asses by taking up 0-2025 of its weight of water.' It 
often occurs in thin seams or payings in rock-s»ilt ; but it also forms large hill-like 
masses, of which the external ]>arti have ]>een converte<l into gypsum. 

Ironstone. — Under this goneml term are included various iron-ores in which the 
pt*roxide, protoxide, carbonate, ^c, are mingled with cla\* and other impurities. They 
have generally l>een de|»osite<l As chemical pi-ecipitates on the bottoms of lakes, nnder 
marshy ground, or within fissures and cavities of rocks. Some iron-ores are associated 
with schistose and massive i<x*ks ; others are found with sandstones, shales, limestones 



* On the mineralogioal natun; of dolomite see 0. Meyer, Z. Deutsch. O'eal. Oes. xxxL 
p. 445, Loretz. op. cit. xxx. p. 387. xxxi. p. 756. Reward, BvlL Acad, Rt^y, Bftg. xhii. 
(1879), No. 5. 

- llotli. <.'h*'m. (w't?ni. i. p. 553. 

^ See (jr. Rose on formation of this rock in presence of a solution of chloride of sodium. 
XmesJnhrh. Miu. 1871, p. 932. Also Bischof. *('heni. und Phys. (leol.* Suppl. (1871), 
p. 188. 



»ART II § vii STRATIFIED CRYSTALLINE ROCKS 153 

nd coals ; while some occur in the form of mineral veins. Those which have resulted 
rom the co-operation of organic agencies are described at p. 146. 

Hfematite (red iron-ore), a compact, fine-grained, earthy, or fibrous rock of a 
•lood-red to brown-red colour, but where most crystalline, steel-grey and splendent, 
rith a distinct cherry-red streak. Consists of anhydrous ferric oxide, but usually is 
lixed i^ith clay, sand, or other ingredient, in such varying i)ro|W)rtions as to pass, by 
isensible gradations, into ferruginous clays, sands, quartz, or jasi)er. Occurs as beds, 
age concretionary masses, and veins traversing crystalline rocks ; sometimes, as in 
Vestmoreland, filling up cavernous spaces in limestone. Is found occasionally in beds 
f an oolitic structure among stratified formations. Some at least of the oolitic or 
•iaolitic ironstones have resulted from the converaion of original grains of calcite in 
rdinary oolites into carbonate of iron which on oxidation has become magnetite, 
icmatite, or limonite. 

Limonite (brown iron-ore), an earthy or ochreous, comjAct, fine-gi*ained or fibrous 
oek, of an ochre-yellow to a dark brown colour, di8tinguisha])le from hrematitc by being 
ijdrous and giving a yellow streak. Occurs in beds and veins, sometimes as the result 
f the oxidation of ferrous carbonate ; abundant on the floors of some lakes ; commonly 
:>und under marshy soil where it forms a hard brown crust upon the impervious subsoil 
bog 'iron -ore). Found likewise in oolitic concretions sometimes as large as walnuts, 
onsistiug of concentric layers of impure limonite with sand and clay {Bohnerz). (See 
.. 146 and Book III. Part II. Sect. iii. § 3.) 

Spathic Iron-ore, a coarse or fine crj-stalline or dull compact aggregate of tlie 
aineral siderite or ferrous carbonate, usually with carl)onates of calcium, manganese and 
oagnesium ; has a prevalent yellowish or brownish colour, and when fresh, its rhombo- 
ledral cleavage-faces show a i>early lustre, which soon disappear as the surface is 
xidized into limonite or haematite. Occurs in beds and veins, esiwjcially among older 
leological formations. The colossal Erzberg at Eisenerz in Styria, which rises more 
han 2700 feet aboA'e the valley, consists almost wholly of siderite.* 

Clay-ironstone (Sphterosiderite), a dull brown or black, compact form of siderite, 
rith a variable mixture of olay, and usually also of organic matter. Occurs in the 
'arboniferous and other formations, in the form either of nodules, wheix* it has usually 
•een deixwited round some organic centre, or of beds interstratified with shales and 
oala. It is more pro[)erly described at p. 147, with the organically derived rocks. 

Magnetic iron -ore, a granular to compact aggi-egate of magnetite, of a black colour 
nd Rtreak, more or less perfect metallic lustre, and strong magnetism. Commonly 
ontains admixtures of other minerals, notably of hivmatite, chrome-iron, titanic-iron, 
yrites, chlorite, quartz, hornblende, garnet, epidote, felsimr. Occurs in beds and 
normous lenticular masses (Stocke) among crystalline schists, likewise in segiegation- 
eins of gabbros and other eruptive rocks ; also occasionally in an oolitic fonn 
probably as a pseudomorph after an original calcareous oolite) among Paljeozoic rocks, 
IS in the so-called **pisolitic iron-ore" of North Wales. Among the Scandinavian 
;neiaaes lies the iron mountain of Gellivara in Lulea-Lappmark, 17,000 feet long, 8500 
eet broad, and 52.5 feet high. • 

Siliewmi Sinter (Geyserite, Kieselsinter), the siliceous deposit made by hot springs. 
nciudiug varieties that are crumbling and earthy, comi>act and flinty, finely laminated 
kod shaly, sometimes dull and opaque, sometimes translucent, with pearly or waxy 
uatre, and with chalcedonic alterations in the older jiarts. The deposit may occur as 
.n incnistation round the orifices of eniption, rising into dome-shaiwd, botr}'oidal, 
oralloid, or columnar elevations, or investing leaves and stems of plants, shells, 
Diiects, Ac, or hanging in pendant stalactites from cavernous sjwices which are from 
ime to time reached by the hot water. When purest, it is of snowy whiteness, but is 
•(ten tinted yellow or flesh colour. It consists of silica 84 to 91 \^v cent, with small 

1 Zirkel, Lehrb. i. p. 34r». 



1 5 4 GEOGNOH Y book ii 



prnpoi-tions of aliniiiiia, fervie oxido, lime, ma^csia, and alkali, and from 5 to S ]«r 
rent of waU;r. (See Book III. Part II. Sect. iii. § 3, i«ar. 6.) 

Flint and Chert have l>eeu already deseril>e<l among the rocks of organic origiu 
inuti^ |». 111). Homstone, an excessively comjuiet siliceons i*ock, usually of some dull 
dark tint, oecui-s in nodular masses or in*cgular l>ands and veins. The name has some- 
times Ikm'u a])plie<l to fine tliuty fomis of felsitc. Vein-Quarts may be alluded to hereu 
a substance which sometimes oecui's in large masses. It is a massiA'c foiin of quarti 
found lilling veins (sometimes many yanis broad) in crystalline and clastic itK^ks ; more 
especially in metamorphic areas. (See Quartz Rocks, p. 179.) 

Some of the other varieties of silica occurring in large masses may be classed as roi'ks. 
Such are Jasper, and Ferruginous Quartz. These, as well as common vein-ijuartz, 
occur as veins traversing both sti-atilied and unstratifiexl rocks ; also as lieds associated 
with the crystalline schists. AVith them maybe grouiM?d Lydian-Stone {LydiU\ Kiftd' 
<t^hivft:i'\ a black or dark -coloured, excessively comjiact, hard, infusible rook with splintery 
fmcture, (H'curriug in thin, sharjOy defined bands, split by cross joints into polygoiul 
fragments, which arc sometimes cemented by Hue layei's of ([uartz. It c<»iisists of an 
intimate mixture of silica with alumina, carbonaceous materials, and o.xide of iron, and 
under the microsi^ojKr shows minute <|uartz-gi-anules with dark amorphous matter. It 
oc^MU's in thin laversor liands in the Silurian and later Pal a'ozoic formations iuterstratififd 
with ordinary sandy and argillaceous strata. As these rocks have not been materially 
altered, the ban«ls of Ijy<lian -stone may l)e of original formation, though the extent to 
which they are often veined with quartz shows that they have, in many cases, been 
|X'rmeated by siliceous water .since their dejKisit. The siliceous i"ocks due to the opera- 
tions of plant and animal life are described on p. 141, also in Book III. Part II. 
Sfct. iii. § 3. 

Some originally clastic siliceous nx-ks have acquired a more or less cr^^stalline 
structure from the action of thermal water or otherwise. One of the most marked 
varieties has W{.'\itiiT\nvA Cnjslallizi'd Smidstone (see p. 132). Another variety, knovn 
;i« (Jiiartzife, is a granular and comjiact aggregate of tjuartz, which will be descrilied in 
connet:tion with the schistose rocks among which it generally jxrurs (p. 180). 

II. Massive — Erufi'ive — Igneous. 

.Vlmost all the members of this importiint suMiAision have been 
|)ro(lucoil from within the crust of the earth, in a molteu condition. 
Xearly all consist of two or more minerals. Considered from a chemical 
p(»int of view, they may be (loscril>ed as mixtures, in different propoi-tions, 
of silicates of alumina, magnesia, lime, potash, and soda, usually with 
magnetic iron and ))hosphate of lime. In one scries, the silicic acid 
has not been more than enough to combine with the different bases; 
ill another, it occurs in excess as free quartz. Taking this feature as 
a basis of arrangement, some petrographers have proposed to divide the 
rocks into an acirl grouj), including such rocks as granite, quartz-porphyry 
and rhyolite, where the percentage of silica ranges from 60 to 75 or more, 
a Inisic group, typified by such rocks as basalt, where the proportion of 
silicii is only about 50 per cent or less, and an intermediate group repre- 
sented by the andesites with a i)roportion of silica ranging between that 
of the other two groups.^ 

^ See a paper on the chemical rrlations of the eruptive rocks by Prof. Kosenbnscb, 
Tst'/hi'fiifik'A Mill. Mitthiil. xi. (1SS9), p. 144, also the paper cjuoted in footnote (2) on p. 
156j and a Memoir on "the origin of Igneous Rocks," by .J. V. bUlings^ PhU, fhc. Wnsking- 
ton, 1S92, p. 00. 



RT II § vii MASSIVE ROCKS 155 

In the vast majority of igneous rocks, the chief silicate is a felspar — 
e number of rocks where the felspar is represented by another silicate 
I leucite or nepheline) being comparatively few and unimportant. As 
e felspars gi'oup themselves into two divisions, the monoclinic or 
thoclase, and the triclinic or plagioclase, the former with, on the 
lole, a preponderance of silica ; and as these minerals occur under 
lerably distinct and definite conditions, other petrographers divide the 
Ispar-bearing Massive rocks into two series: (1) the Orthoclase rocks, 
.ving orthoclase as their chief silicate, and often with free silica in 
cess, and (2) the Plagioclase rocks, where the chief silicate is some 
ecies of triclinic felspar. The former series corresponds generally to 
e acid group above mentioned, while the plagioclase rocks are in- 
rmediatc and basic. It has been objected to this arrangement that 
e so-called plagioclase felspars are in reality very distinct minerals, 
th proportions of silica, i-anging from 43 to 69 per cent; soda from 
to 1 2 ; and lime from to 20.^ In addition to the felspar-rocks, 
ere must be noted those in which felspar is either wholly absent or 
aringly present, and where the chief part in rock-making has been 
ken by nepheline, leucite, olivine, or serpentine. 

From the point of view of internal structui*e, a classification based 
K)n microscopic research has been adopted by other ^vriters, who 
cognise three leading types of micro-stnicture — Granular, Porphjritic 
id Glassy^ or Holocrtjstalline, Hemicrystalline and Vitreous. MM. Fouqu6 
id Michel-Levy, pointing out that most eruptive rocks are the result 

successive stages of crystallization, each recognisable by its own 
laracters, show that two phases of consolidation are specially to be 
«erved, the first (porphyritic) marked by the formation of largo 
ystals (phenocrysts) which were often broken and corroded by 
echanical and chemical action within the still unsolidified magma ; 
e second by the formation of smaller cry stills, crystallites, Arc, which 
e moulded round the older series. In some rocks the former, in others 
e latter of these two phases is alone present. Two leading types of 
ructure are recognised by these authors among the eruptive rocks. 

Granitoid, where the constituents are mainly those of the second 
K)ch of consolidation, but where neither amorphous magma, nor 
ystallites are to be seen. This structure includes three varieties, (a) 
le granitoid proper, having crystals of approximately equal size; (b) 
gmaiiM^ where there has been a simultaneous crystallization and 
gular arrangement of two constituents ; {c) ophitic^ in which the felspars 
e ranged parallel to one of their crystalline faces, forming a kind of 
ansition into microlitic rocks. 2. Trachytoid, distinguished by a 
ore marked contrast between the crystals of the first and second con- 
•lidation, the usual presence of an amorphous magma, and the fluxion 
ructure. Three varieties are named : (a) petrosilicemis, with trains and 
iherulites of a finely clouded substance characteristic of the more acid 
►cks ; (b) microlitir, characterised by the abundance of microlites of 

* Dana, Amer, Journ. Sci. 1878, p. 432. Tlie modern methods of separating the fel- 
ars remove some of the difficulty above referred to. 



1 56 OKOfrXOS ] ' BOOK II 



felspars iiiul other minci*als ; (c) liireou^, derived from the two foregoing 
varieties by the predominance of the amorphous paste. ^ 

It is common to introduce a chronological element into the classifica- 
tion of the massive rocks and to divide them into an ancient (Palaeozoic 
and Mesozoic) and modern (Tertiary and recent) series. Certain broad 
distinctions can doubtless l)e made between many anpient and modem 
eruptive rocks ; but, for reasons already stated, it seems inexpedient, in 
the present sttite of our knowledge, to employ relative antiquity (which 
must be determined by a totally distinct branch of geological inquiry, 
and may be erroneously determined) as a basis of petrographical arrange- 
ment. - 

In the following arrangement the three-fold division first mentioned 
al)ove is adopted, according to the relative abundance of silica : Ist Acid, 
2nd Intermediate, 3ixl Bjisic. In each of these series there is a range of 
structure from completely crystalline to completely glassy. The hole- 
crystalline rocks are as a rule the deep-seated representatives of each 
series, while the vitreous and semi-vitreous are those which have either 
]»een erupted to the surface or have been connected with volcanic rather 
than plutonic action. No system of classification yet proixwed can avoid 
incongruities, and it must be rememl>ered that the hard and fast lines of our 
nomenclature do not represent any reiilly abrupt demarcations in nature. 
As one rock graduates into another, our terminology should be elastic, so 
as to include such transitional forms. 

i. Acid Series. 

In thij* family the silicic acid has been in such excess as often to so|iarate out iu the 
foiiM of flee quartz. Sometimes, as in «(ianite, it lias not assumed a definitely crystal- 
lized form, but is moulded round the other crystals as a later stape of consulidatioii. In 
other iwks (quartz-i>orphyry, &e.} it occurs as a ]>rcMluct of earlier consolidation, and 
oftJ'u assumes perfect crystallo^a])hic contdui-s, occurrin;^ even in double pyramids 
The t<*xture of the ro<rks is (1; holwrystalline or crystalline-^anular (granitoid) SJ 
ty]>ieally dev«'lojK*d iu granite : (2) hemi-crystalline (ijorphyritic, tracliytoid), as in 
quartz- porphyry or felsite ; (:3) vitreous, as in obsidian. 

Granite.''— A thoroughly riystalline-gi-anular admixture of quartz, felspar, and mica, 
in partii'les of tolerably unifonn size (Fij^. 15 and *29;. Tlie feUiiar is chiefly 
white or pink orthoclase, but triclinic felspars (oli^joclase and albito) may often be 
observed in smaller rpiantity, frequently distin^iishable by tlieir fine striatlon and 
more waxy lustre. Microcline is not infrequent, as well as the interctystallization 
of orthoclase and pla;j;ioelase (Perthite). The niiea may Ik? the ]»ota.<ili (muscovite) 



* * Minertdogie Micrographique,' p. ITiO. 

" For a t^diulnr arran^renient of the massive (eruptive) rocks and critical remarks on 
their classification, see Hosenbii.^ch, Xitus Johrh. 18S*J, ii. ]». 1. 

^ On the structure of granite, see the manuals of Zirkel and Rosenbusoh and the niemoin 
there cited ; also Zirkel's 'Microscoj*. Petrography,' 187H, p. 39 ; Philli{>s, Q. J. Oeol. Soe. 
XX xi. p. 3.30 ; xxxvl. p. 1. ,J. (.'. Wanl, op. eit. p. 509 ; an<l xxxii. p. 1. King's 
'Systematic Geology' (vol. i. oi Exphn', AOfh PonOU'l), p. Ill <»/ st^q. Michel-Levy, BhU. 
S,^. iit'itf. FrancCy 3nl ser. iii. p. 199. Rosenbusch, Xeituc/t. Ih'iitsc/i. f'Otf. (resell, xxriii. 
(1876), p. 369. H. Mtihl, yi/t. MnJi. Xnt. xxiii. p. 1 et ser/, J. Lebmanu, * Unter- 
suchungen ul)er die Kiitstehung der Altkrystallini.schen Schiefergesteim*,' 1S84, p. 8. W. J. 
Sollas, TitiHs, Ro;i. Irish Acad. xxix. Part xiv. (1S91). 




PART II § vii MASSrFE ROCKS— dRAXITE 1S7 

variety, usually of a white silvery aspect, but luore I'uliimaiily biotite or ottiei' lUrk 
brown or blatk VBiiety. The <|iittrti may lie obseived to foim a kiiiU of paste or 
maf^mii wrapping routiil tlie other iDgredients. 

Only in cavities of the granite do the coii1]hiu- 

ent niineraU occnr in iudependent well-formeil 

crystkla, and there too the accessory luiuerahi 

(heryl, topaz, tourmaliue, garnet, Ac.) are 

chiefly found. 

From a microHcopic examination of granite, 

it was formerly inferred that the rock baa a 

thoroughly crystalline structure, with nomega- 

BMpic gronnd-maaa, iior microscopic base of 

any kind between the crystals or crystalline 

individuala. More recent and exhaustive study 

of the subject, however, has led to the con- 

cliuiion that though tiothitig like a vitreous, 

or even j>ori>hyritic, gronud-moss can be de- 
tected, there is yet sometimes discernible an •■'«• ^■'■— ""''^''J'l'*"'"^'*'™'''"''" "''''"""'c 

analogous kind of entirely CTystatline magma, 

in which the crystals or crystalline debris of the rock are embedded, and in which they 

an partially dissolved. Having regaiil to the relations between this magma and its 

cnclwed mineials, M. Michel- Liivy has observed tliat microscopic examination points to 
a distinction between granites in which the iiuartz is more recent than the other con- 
■titufnta and has consolidated at once, and those in which there are remains of earlier 
bi-]iynunidal quartz. He dutinguislies these two si'iics as (A) Ancient granitei', 
canu{>Mied of black mica, hornblende, oligoclase, and oi'thoclase, forming a crystalline 
debris embedded in a more recent crystalline magma of oi'thoclase and •[uart:', 'Jt) 
Porjiliyroid granites, generally liner in grain than tlie |ireccding, and furtlier disCin- 
guiabed by the occurrence of bi-|iyramidal crystaU of quartz (which made tbcir apjiear* 
ance between the old felspar and the recent orthoclaxe), and of ■ notal)le ijnantity of 
white mica (rare among the ancient granites) ]«s1erior in advent even tii the more 
Tvcent quartz. ' 

Amoug the coni|Kiucnt minerals of gmnite, the qniLitz jircsents s^iecial interest 
under the microscope. It is often found to be (\lll of cavities containing li<)uid, some- 
timeH in such numbers as to amount to a thousand millions in a cubic inch and to give 
a milky turbid aspect to the mineral. Tlie tiqnld in tliese cavities ajiiiears usually to 
be water containing sodium and |>otastiiuin chloriiles. with sulphates of these metals and 
of calcium (p. IIOJ. 

The mean of eleven analyses of granites made by Di'. Kangbton gave the following 
average comjiositian ; silica, 72-07; alumina. U'Hl : peroxide of iron, 2-22 ; jiotssh, 
5-11 ; soda, 2-79: lime, rS3 ; magnesia, O'SS ; loss by ignition, 1-09; total, 10005, 
with a mean speciRc gisvity of 2-flti. 

Most large masses of granite present differences of texture in different ]iarts of their 
area. Some of rhese variations depend on the relation of the mass to tbe surrouniling 
rocks (see pottra, p. 505). Others may occur in any ]iortion of a granite liosii, and 
have been produced by the circumstances in which the mass consolidated. Some 
granites are marked by the occurrence of the cavities alH>ve referred to wbere the 
individual minerals hare had room to ussume Hbarjily delined ciystolline forms. 31any 
granites are apt to be traversed by veins, soinvlimes due to a segregatian of the 
sarroundiDg minerals in rents of tlie original )>asty magma, sometimes to it jirotiiision 
of a less coarsely ciystaUine (micro-granitic, felsitic) mateiial into the main rm-k (Fig. 
30>, Some of the more im[iortant of tbesu varieties bit distitigui shell by speiint names. 

' BvU. Soc. (fill. Fnince, 3rd ser. iti. [Ii)75}, )>. I'M. 




vi' i.'iystallizml togptLor so u to 
-ii-litutioii of their lolifcvr BXH IB 
oiii- {{eiieral (lirwtiou, a» Qttj 
oivHjiwialty apt to do iti itegnfi' 
lioti-vciiM, the rock in tmud 
IVgTiintiU'.' Olir of tin- bum 
imiTpatiiiB Kirucliiml v»rieti« b 
till' I'onii i)f ]H-(nuatit« tenntd 
r>rii)iliU- Urauite. in w-Mch 
thp uni'ntatiun at tliv i|iiBrti ud 
fi'lH]*!]' is HinEnlarlf well devrl- 
«]jv>l iFig. 3li. Ttie >iiiarti b«t 
aHHinniil the shajie of long in- 
]KTfi'tt cnhiniiiar HlielU, plmcnl 
jKirnUcI t'l eai'h other and eiieloMil 
uithiii llie orthoclaae. so that* 
ti-Hiisi-trse xwtioii liean noaif 
iiwiiililuiice to Hebrew MTitiajt 
Tilt' t nomiiiei'alii have ciyatallivl 
tofS'thiT, H'ith their priiiei|ial am 
jiarutlvl. This iiitcrgrowth tmni 
tn sliMH- tliat there tould haxr 
Ih'cii tilth' or no internal mon- 
iiieiit it( tlie veini. ID which itn 
fi¥i|ili'iit1y oci'urs, when the eoU' 
|H>iii'iit niiiiersht •ntunieil thrir 
tryntnllilie forraB. Where the 
iiiter^^wtli is oil a minute icak 
it iskliomi as niieroiiegmatite, 
and it rtiniiH the li«se of tlie rockbt 
u-liiuli tlie uiiine of Grauoplijre 
finiiiil of a (iranite bwooiiig 




tiup-gmiiieil. Ihu I'liutaiuiii); Urjjv Hcuttered fctH)iar I'l^-slais. .Suclt a rock may liv tcrmtd 
a imiriJiijrUic iiriiiiili: Soniu Kraidtes b>iuiiii'1 id dicluwil i-rystalline concretioui ot 



' i\a an sJiiiiralile iind rxliniistivi' u 
luiuernLs iu Sontlwni Nurwaj'. see the { 



ot the Pcjiiimliti- vtiuii, and their MMKiated 
ouogniiili liy l-rof. W. C, Briigger in Gioth'* 



PART II S vii MASHIVE ROCKS— GRANITE 159 



Vagmeuts. These are sometimes mere segre^tions of the materials of the granite, when 
iiey are usually ovoid in form and porphyritic in structure ; in other cases, they are 
'ragmenta of other rocks, and are then commonly schistose in structure and irregular in 
bnn.^ In rare exam])les the component minerals of granite have crystallized with a 
sdial concentric arrangement into rounded ball-like aggregates (spheroidal, orbicular 
granite). ^ In the centre, as well as round the edges of large bosses of granite, the 
oiiierals occasionally assume a more or less perfectly schistose arrangement. When this 
akes place, the rock is called gneissose or gneiss granite. (See Book IV. Part VII.) 

Differences in the proportions or nature of the component minerals have likewise sug- 
;e8t«d distinctive names. Of these the following are the more imiK)rtant : Granitite, 
biotite granite) — a mixture of pink orthoclase and abundant oligoclase, with a little 
uartz, some blackish green magnesia-mica, and occasionally with hornblende or augite. 
lorn blende -granite— a rock with hornblende added to the other normal constituents 
f grmnite, and usually poorer in quartz than normal granite. A well-known variety 
ccors at Syene in Upi)er Egypt, whence it was obtained anciently in large blocks for 
belisks and other architectural works. The well-known Egyptian monoliths are made 
f it. It was called by Pliny "Syenite," — a name adopted by Werner as a general 
esignation for homblendic granites without quartz. The rock of Syene is really a horn- 
lende-biotite-granite. Augite-granite — a variety in which augite occurs with black 
lica. Tourmaline granite — a granitite with disseminated tourmaline. Greisen 
-a rare granitic rock from which the felspar has disap^^eared, found in some granite 
jstricts, especially in those wherein mineral-veins occur. A p 1 i t e — a fine-grained mixture 
f quartz and felsi»ar, which have not infrequently intergrown (micropegmatite) ; found 
specially in veins in granite. " Elvan " is a Cornish term for a cr3rstalline-granular 
nixtnre of quartz and orthoclase, forming veins which proceed from granite, or occur 
•nly in its neighbourhood, and are evidently a^ociated with it.^ Under the name 
rranulite M. Michel -Le>'y includes certain tine -grained gi-anites with white mica, 
rbich to the naked eye appear to be composed entirely of felspar and quartz, or 
f felnpar alone, though both mica and quartz aj>pear in abundance when the rocks 
jne microscopically examined. He includes in this category most of the rocks of the 
UpB deserib^ as ** protogine." 

Surrounding large masses of granite there are usiuilly numerous veins, which consist 
»f granite, quartz -porphyry, felsite, or sometimes even sphenilitic material (Mull). 
rhere can be no doubt that these finer-grained protrusions really proceed from the ciys- 
alline granite mass. Lessen has shown that the Bode vein in the Harz has a granitoid 
entiVy with compact por]>hyry sides, in which he found with the micrascoiH' a tnie gla.s.sy 
laae.^ Sometimes the rocks associated in this way with granite differ in composition from 
he main granitei Tourmaline is one of the characteristic mhierals of gianite-veins, 
hou^ lev observable in the main body of the rock ; with quartz, it forms Schorl-rock. 

' Gimnite weathers chiefly by the decay of its felsjMirs. These are converted into 
laolin, the miea becomes yellow and soft, while the ({uartz stands out scarcely affected. 
rhe granite of the south-west of England has weathered to a depth of 50 feet and 
tpwards, so that it can be dug out with a si>ade, and is largely us(>d as a source of 
lonselain-elay. 

<^6ranite ooonrs (1) as an eruptive rock, forming huge bosses, which rise through other 
ormatious both stratified and unstratified, and sending out veins into the surrounding' 
nd overlying rocks, which usually show evidence of much alteration as they apjironch 

» J. A. PhiUipe, Q. J, 0(4)1, Soc, xxxvi. (1880), p. 1. 

* W. C. Brogger and H. Backatrom, Oei>i. StockJudm FUrhandl. ix. (1887), p. 307. Hatch. 
>Hart, Journ. OtU. Soc. xliv. (1888), p. 548, and authorities there citetl. 

» J. A. Phillips, Q, J. Gtol. ^Soc. xxxi. p. 334. Michel-Levy, BvU. Soc. GitA. Fro.na\ 
i. Srd ser. p. 201. 

« ZcUseh, Deuiseh, Oeol, Ges, zxvL (1874), p. 856. 



1 60 GEOGXOS Y book n 

the granite; (2) connected witli tnie volcanic rocks (as in the Tertiary granophyret of 
Mull and Skye), and forming, ]>erha|)8, the lower yiortions of masses which flowed ou 
at the surface as lavas, (jlranite is thus a decidedly phUonlc rock ; that U, it hM 
consolidated at some depth l)eneath the surface, and in this resiiect difl'erB from the 
suiieriicial vulcanic rocks, such as lavas, which have flowed out above ground fna 
volcanic oiifiees. 

Quartz-Porphyry (Microgi-anite, Eurite).* — A tine-grained microgranitic groimd- 
mass, com]iosed mainly of fels^iar and quartz, through which are usually scattered coi' 
si>icuous i)orphyritic crystals of one or other or l»oth of the same minerals. 

To the naked eye the ground-mass varies from an exceedingly compact texture toose 
where ahundant minute crystals can be detected. Of the por]>hyritic constituents Xht 
({uartz occasionally occurs in bi-pyramidal crystals : the felsiiar is usually ortbodiM^ 
while black mica occasionally ap] Heat's. Under the microscojK? the stnicture of the rock 
is found to be microgranitic, with frequently a micropegmatitic arrangement of the 
quartz and felsjjar (gi-anophyre). 

The flesh -red (|uartz-jK)rj>hyry of Dobritz, near Meissen, in Saxony, was found fcy 
Reutzsch to have the following chemical composition : Silica, 76«92 ; alumina, 12*89; 
]M>tash, 4-27 ; soda, 0-68; lime, O-eJS ; magnesia, 0-t)8 ; oxide of iron, 1-15; watff, 
1*97 ; total, 99'54, — specific gravity, 2«49. 

The coloui-s of the rock dei>end chiefly upon those of the fels|iar, — \m\e flesh-Ewit 
reddish-brown, purple, yellow, bluish or slate-grey, ]wissing into white, being in difiv* 
ent places characteristic. It will be observed in this, as in other rocks containing mwh 
felsiMir, that the colour, besi<les deiicnding on the hue of that mineral, is greatly re- 
lated by the nature and stage of decomiK>sition. A rock, weathering externally with a 
pale yellow or white cnist, may be found to be dark in the central undecayed portioiL 
AVhen the base is very com[»act, and the felspar-ciystals well-defined and of a diflferat 
colour from the base, the nx'k, as it takes a good ]K)lish, may be used with effect sin 
ornamental stone. In popular language, such a stone is classed with the ''marbkt,* 
under the name of "i»orphyry." 

The Quartz-porj»h3'ries occur (1) with j>lutonic rocks, as em ptive bosses or veili, 
often associated with gi-anite, from which, indeed, they may be seen to proceed directly ; of 
freijuent occurrence also as veins and iiTcgularly intrudetl masses among highly conTOl- 
uted roi'ks, ej*i)ecially when the^se have been more or less metamorphosed : (2) in the 
chimneys of old volcanic orifices, forming there the ** neck" or plug by which a ventii 
filled up ; and (3) as bosses sometimes of large size which have been protruded in oonnw* 
tion with volcanic action. Between the gi-anoj»hyres which are charaetcriaed by a micro> 
]>egmatitic structure and the felsites or ancient rhyolites there is a close relatiflB. 
4,Hiartz-iK>rphyries are abundant in Britain among formations of Lower Silurian, Old 
Red Sandstone and Lower Carboniferous age. In the Inner Hebrides they occur in laigl 
bosses or domes (gianophyre) rising through the older Teitiary basaltic plateau. 

Many of the rocks called '* quartz-porphyry " are not mici'ogranitio but have tfai 
*"felsitic" structure arising from the devitrification of ancient forms of rhyolito 
<sce i». 161). 

Bhyolite''^ (Li|»aritc, Quartz-trachyte)— a rock having a com^iact liale-grey, yellowith, 
gieenish or reddish ground - mass, sometimes with glassy patches and layers oftn 
showing j>erfect fl(>w-structure, not infrequently also with spherulitio and peiiitk 
stnictures, and with ciystals of orthoclase (sanidine), granules of quartz and minute 
ciystals of black mica, augite, more rarely hornblende. Considerable diversity ezisti ii 
the texture of the rock. Frequently it is finely cavernous, the cavities being lined 
with chalcedony, (juartz, amethyst, jasjier, &c. Some varieties are coarse and gnmitoid 

^ Zirkel, * Microscop. Petrog.' p. 71. See particularly Koseubusch, '3iik. Phy8/iLp.50. 
- On rliyolite see Richthofen, Jahdt. K. K. UcoL Heichsanst, zi. 156. Zirkel, *Mienk 
Petrog.* p. 163. King, *Exi)lor. 40th Parallel,' vol. i. p. 606. 



PART u § vii MASSIVE ROCKS—RHYOLITE 161 



in character. Intermediate varieties may be obtained like the quartz-[)ori)liyries, and 
these pass by degrees into more or less distinctly vitreous rocks. Throughout these 
gradations, however, which doubtless represent different stages in the crystallization of 
an original molten glass, a characteristic ground -mass can be seen under the micro8co]>o 
having a glassy, enamel-like, |)orcellaneous, microlitic character, with characteristic 
spherolitic and fluxion structures. In the ([uartz, glass-inclusions, having a dihexaliedral 
form, may often be detected ; but liquid inclusions are absent. An analysts by Vom 
Rath of a rhyolite from the Euganean Hills gave — silica, 76*03 ; alumina, 13*32 ; soda, 
5*29; potash, 3*83; protoxide of iron, 1*74; magnesia, 0*30; lime, 0*85 ; loss, 0-32: 
total, 101*68,— specific gravity, 2*553. 

The perlitic structure is so characteristic of this rock that the varieties which specially 
exhibit it were formerly regarded as a distinct rock-species under the name of Perlite or 
FearfsUme. As the name indicates, the structure presents enamel -like or Wtreous 
globules which, occasionally assuming polygonal forms by mutual pressure, sometimes 
constitute the entire rock, their outer portions shading off into each other, so as to form 
a compact mass ; in other cases, se[)arated by and cemented in a compact glass or 
enameL They consist of successive very thin shells, which, in a transverse section, arc 
seen as coiled or spiral rings, usually full of the same kind of hair-like crystallites and 
ciyataUi as in the more glassy i)arts of the rhyolite (Fig. 9). As these bodies both 
singly and in fluxion-streams traverse the globules, the latter may be regarded as a 
structure develo[)ed by contraction in the rock, during its consolidation, analogous to 
the concentric spheroidal structure seen in weathered basalt (Fig. 94). Among these 
concentrically laminated globules true spherulites occur, distinguished by their internal 
radiating fibrous structure (Figs. 7, 17). 

Rhyolite is an acid rock of volcanic origin. It forms enormous masses in the heart 
of extinct volcanic districts in Eui-o^ie (Hungary, Euganean Hills, Iceland, Li])ari), 
and in Xorth America (Wyoming, Utah, Idaho, Oregon, California). 

Nevadite — a variety of rhyolite named by Richthofen from its development in 
Nevada, and characterised by its resemblance to granite, owing to the abundance of its 
porphyritic crystals, and the relatively small amount of ground-mass in which they are 
imbedded. The granitoid as^iect is external only, as the ground-mass is distinct, and 
varies from a holocrystalline character to one with abundant glass, and the texture 
ranges from dense to [lorous.^ 

Felsite (Felstone). — Under this name a large series of rocks has been grouiM?d which 
appear for the most i>art to have been originally %itreous lavas like the rhyolites, but 
which have undergone complete devitrification, though frec^uently retaining the |)erlitic, 
spherulitic, and flow-structures. They varj' in colour from nearly white through shades of 
grey, blue and red or brown to nearly black, often weathering with a white crust. 
They are cloee-graine<l in texture, often breaking with a sub-conchoidal fracture and 
showing translucent edges. Por])hyritic fels|)ars (lx)th orthoclase and plagioclase) and 
blebs of quartz are of freijuent occurrence. The flow -structure is occasionally strongly 
marked l^ bands of different colour and texture, sometimes curiously bent and curled over, 
indicating the direction of movement of the still unconsolidated rock. The sphemlitic 
structure also may be found so strongly marked that the individual spherules measure an 
inch or more in diameter, so that the rock seems comix)sed of an aggregate of balls, 
and was formerly mistaken for a conglomerate {Pyrom^ride).' Under the niicroscoi>e many 

* Hague and hidings, Avier. Journ. ScL xxvii, (1884), p. 461. These authors dis- 
tinguish between Nevadite and Liparite, the latter being characterised by the small 
number of porphyritic crystals imbedded in a relatively large amount of ground-inass, 
which, as in Nevadite, may be holocrystalline or glassy. Tliey also distinguish LithoUfa! 
Rhyolite and Hyaline RhydUe as ad<litional varieties. 

' On nodular feldtes see G. Cole, Quart. Juvni. O'enl. Soc. xli. (1885), p. 162 ; xlii. p. 
183 ; Hiss Raisin, op, dt. xlv. (1889), p. 247. Harker '' Bala Volcanic Rocks," 1889, p. 28. 

M 



1 62 GEOGNOS Y book u 

of the typical structures of rhyolite can be detected in felsit^. The grouud-inass of tlie« 
rocks has given rise to much discussion, but it is now generally recognised as a more or leai 
altered condition of the dev-itritication of an original vitreous mass (p. 117). Secondiiy 
changes have in large measure destroyed the original microlitic structure, but traces of it 
can often I>e found while the spherulitic and ])crlitic forms frequently remaiu almost u 
fresh as in a recent rock. Felsites with a large proportion of alkalies, especially soda, 
have been called Keratophyrcs.^ 

Felsites have Iteeu found abundantly as intcrbedded lavas with tufis and a^lomei^ 
ates associated with Silurian and older rocks in Wales and Shropshire.' Soda- felsites or 
kerat-oi>hyrcs have l>een found to play a considerable |)art among the materials erupted 
by the Lower Silurian volcanoes of the south-east of Ireland.' 

The vitreous acid rocks form an interesting group in which wc may still detect wlist 
was probably the original condition of at least the rhyolites and felsites. Every grads- 
tion can l)e traced from a ))erfect glass into a thoroughly devitritied and even crystalline 
rock. As ah'cady remarked, the original vitreous condition of rhyolite can still be seen 
even with the naked eye in the clots and streaks of glass that occasionally run throng 
it in the direction of its ilow-structure. Various names have been given to the gUsiy 
rocks, of which tlie chief are obsidian. ]iitchstone and pumice. Tliese, however, are noC 
to be regarded as distinct nK>k-s[)ecies but rather as the glassy condition of diflereat 
lavas. 

Obsidian (rhyolite-glass) — the most i»erfect form of volcanic glass, externally resem- 
bling bottle glass, having a jierfect conch oidal fracture, and breaking into sharp splinten^ 
transparent at the edgcts. Its colours are black, brown, or greyish-green, rarely yellow, 
blue, or red, but not infrequently streaked or banded with jmlcr and darker hues. A tfain 
slice of obsidian pre])arc(l for the microsco|)e is found to be very pale yellow, brown, 
grey, or nearly colourless, and on being magnified shows that the usual dark colours an 
almost always produced by the ] presence of mi nut« oj)aque crystallites, which present 
themselves as black oi)aque trichites, sometimes beautifully arranged in eddy-like lines 
showing the original fluid movement of the rock (Fig. 19) ; also as rod-like transparent 
microlites. They occasionally so increase in abundance as to make the rock lose the 
aspe<2t of a glass and assume that of a dull flint-like or enamel-like stone. This devitri- 
fication can only l)e properly studieil with the microscope. Again, dull grey enamel- 
like s])lierulites api»ear in some parts of the rock in great abundance, drawn out 
into layers so as to give the rock a tissile structure, while steam- or gas-cavities likewise 
occur, sometimes so large and abundant as to impart a C(>.llular as])cct. Tlie occurrence 
of abundant sanidine cr^-stals gives nse to Porphyritic Obsidian. Many obsidians, from 
the increase in the nuniljer of their steam -vesicles, pass into }mmice. Now and then, 
the st4;ani-porcs are found in enonnous nimil)ers, of extremely minute size, sa in 
an obsidian from Iceland, a plane of which, about one st^uare millimetre in sin, 
has been estimated to include 800,000 iK)res. The average chemical composition of 
obsidian is — silica, 71-0 ; alumina, 13-S ; i>otash, 4*0 ; soda, 5'2 ; lime, 1-1 : magnesis, 
0*6 : oxides of iron and manganese, 8*7 : loss, 0*6 (little or no water). Mean speciiic 
gravity, 2*40. Obsidian occurs as a product of the volcanoes of late geological perioda 
It is found in Li)»ari, Iceland, and Teneriifc ; in North America, it has been erupted 

^ Giimbel, 'Palaeolit. Eniptivgest. Fichtelgebirg. ' (1874), p. 43. Rosanbnsch, 'Mlkros- 
kop. Physiog.'ii. 434. 

''^ Mr. Alli)ort descri1>ed some ancient forms of perlitic structure from Shropshire, in 
what were probably once ordinary rhyolites, Q. J, Gcoi. «S(t. xxxiil. p. 449 ; and Mr. Rntky 
showed the presence of the same structure among the Lower Silurian lavas of North Wales. 
Op. cU. XXXV. p. 508. 

' F. H. Hatch, Man. Geol. Surv. Ireland, Explanation of Sheet 130 ; Oeol. Mag, 1889, 
p. 70. 



PART u § vii MASSIVE ROCKS— SYENITE 163 

from many points among the Western Territories ; ^ it is met with also in New 
Zealand. 

Pitchstoneis a name given to the less perfectly glassy acid rocks, which are distin- 
guished by a resinous or pitch-like lustre, and internally by a more advanced development 
of microlites than in obsidian. They thus represent a further stage of devitrification. 
These rocks are easily frangible, breaking with a somewhat splintery fracture, translucent 
on thin edges, with usually a black or dark green colour, that ranges through shades of 
green, brown, and yellow to nearly white. Examined microscopically, they are found 
to consist of glass in which are diffused hair-like, feathery and rod-shaped microlites, 
or more definitely formed crystals of orthoclase, plagioclase, quartz, hornblende, augite, 
magnetite, &c The pitchstone of Corriegills, in the island of Arran, [)resents abundant 
green, feathery, and dendritic microlites of hornblende (Fig. 14).^ Occasionally, as in 
Airan, pitchstone assumes a spherulitic or perlitic structure. Sometimes it becomes 
porphyritic, by the development of abundant sanidine crystals (Isle of £igg). 

Pitchstone is found as (1) intrusive dykes, veins, or bosses, probably in close con- 
nection with former volcanic activity, as in the case of the dykes, which in Arran 
trmyerse Lower Carboniferous rocks, but are probably of Miocene age, and those which 
in Meissen send veins through and overspread the younger Palfpozoic felsite-porphyries : 
(2) sheets which have flowed at the surface, as in the remarkable mass forming the 
Soair of Eigg, which has filled up a river-channel of older Tertiary age.' 

Pumice (Ponce, Bimstein) — a general term for the loose, 8i)ongy, cellular, filamentous 
or froth-like parts of lavas. So distinctive is this structure, that the term pumiccotis 
has oome into general use to describe it. There can I)e no doubt that this froth-like 
rock owes its peculiarity to the abundant esca[)e of steam or gas through its mass while 
still in a state of fusion. The most perfect forms of pumic« are fouud among the acid 
lavas, but this type of rock may be met with in the other groups. Microscopic 
examination of a rhyolitic pumice reveals a glass crowded with enormous numbers 
of minute gas- or vapour-cavities, usually drawn out in one direction, also abundant 
crystallites like those of obsidian. Owing to its porous nature, pumice possesses great 
buoyancy and readily floats on water, drifting on the ocean to distances of many 
hundreds of miles from land, until the cells are gradually filled with water, when 
the floating masses sink to the bottom.^ Abundant rounded blocks of pumice were 
dredged up by the Challenger from the floor of the Atlantic and Pacific Oceans. 

ii. Intermediate Series. 

In this series, the average percentage of silica is considerably less than in the acid 
series (56-66 per cent). Free quartz is not found as a marked constituent, although 
occasionally it occurs in some quantity, as microscopic examination has shown in the 
case even of some rocks where the mineral was fonnerly believed to l)e absent. A range 
of structure is displayed similar to that in the acid series. The thoroughly cr^'stalline 
▼arieties are typified by syenite (and diorite), rei)resenting the granites of the acid 
rocks, those which possess a porphyritic ground-mass by orthoclase- porphyry, trachyte, 
and andesite, answering to quartz-porjjhyry and rhyolite, while the vitreous condition 
is represented among ^e trachytes and andcsites by dark glasses of the obsidian and 
pitchstone types. 

8j«dt«. — ^This name, fonnerly given in England to agranite with hornblende replacing 

' For an account of the obsidian of the Yellowstone Park see J. P. Iddings, 
Ith Rept.U, S. OeoL Surv. (1885-86), p. 255 ; consult also Zirkel, *Microscop. Petrog.' 

- See F. A. Gooch, Min. MUtheU. 1876, p. 185. Allport, Qeol, Mag. 1881, p. 438. 

> Quart. Joum, GeoL Soe, (1871), p. 303. 

* On porosity, hydration, and flotation of pumice, see Bischof, ' Chem. und Phys. Geol.' 
sappL (1871), p. 177. 



1 64 GEOGNOS Y booe U 

mica, is now restricted to a rock consisting essentially of a lioloerystalliiie mixture of 
orthoclase and hornblende, to which plagioclase, biotite, angite, magnetite, or quartz msj 
be added. As already mentioned, the word, first used by Pliny in reference to the rock 
of Syene, was introduced by Werner as a scientific designation. It was applied b^' liin 
to the rock of the Plauenscher-Grund, Dresden ; lie afteni'ards, however, made thit 
rock a greenstone. The base of all syenites, like that of granites, is thoroughly cryatal- 
line, without an amorphous ground-mass. The typical syenite of the Plauenacher-Grond, 
fonnerly described as a coarse-grained mixture of flesh-coloured orthoclase and bhek 
hornblende, containing no quartz, and with no indication of i)lagioclase, was regarded 
as a nonnal orthoclase-hornblende rock. Microscopical research has, however, shovi 
that well-striated triclinic fels[>ar8, as well as quartz, occur in it. Its composition is ^— 
silica, 59-83 ; alumina, 16-85 ; protoxide of iron, 7-01 ; lime. 4-43 ; mai^esia, 2-61 ; 
l>otash, 6-57; soda, 2-44; water, etc., 1-29: total, 101-03. Average siiecific gravity, 
2-75 to 2-90. 

Syenite is of much less fre<iuent occurrence than granite. While always thorouji^ilj 
granitic in structure, it varies in texture from coarse gi-anular, where the individinl 
minerals can readil}' be distinguished by the naked eye, to conqiact. Among iti 
accessory minerals of common occurrence may be mentioned titanite (spheue), quarts 
ajmtite, e[>idotc, orthite, magnetite, pyiite, zii*con. The pit*rlominance of one or men 
of the ingredients has given i-ise to the? soimration of a few varieties under distinetin 
names. In the typical syenite, the dark silicate is almost wholly hornblende ; some- 
times there are to be found traces of augite within the hornblende, indicating that the 
former mineral was the original constituent and has been changed by paramorphiaB. 
Where the ferro-magnesian silicate is mainly augite, as in the well-known rock of 
Monzoni, the r(K'k is tcnned Augite- syenite or Monzonite ; where brown mioi 
preilominates it gives rise to Mica -syenite or Minettc. 

Elaeolite-syenite {XepheUac-sitf.nU^^ is a granitoid rock, characterised by tiie 
association of the variety of uepheline known as elaeolite with ortluKrlase, and vitk 
minor proiwrtions of plagiocla-ne, microcline, hornblende, augite, biotite, sodalite, zirecai, 
and spliene. It is distinguished by the rare mineral.s, upwards of fifty in number, which 
it contains, and in which some of the rarer elements are combined, such as thorimn, 
yttrium, cerium, lanthanum, tantalum, niobium, zirconium, &c. It is typically de- 
veloi)ed in Southern Xonvay (Hrevig, Laurvig). Where zircon enters as an abundant 
constituent the rock is known as Zircon-syenite. Foyaite is the name given to 
a hornblendic variety found at Mount Foya, Portugal : Miascite is a variety nith 
abundant mica, found at Miusk ; Ditroite, containing sodalite, spinel, etc., OGcnnat 
Ditro in Tran.sylvania. 

Orthoclase -Porphyry (Micro-syenite. Quartzlcss- porphyry, Orthophyre) stands to 
the syenites in the same relation that quartz-ix)rphyry or micro-granite does to the 
granites. It is comiM)sed of a comi>a(;t micro-granitic ground-mass, with little or no 
free quailz, but through which arc usually scattered numerous cr^'stals of orthoclase^ 
sometimes also a triclinic fels[>ar, black hornblende and glancing scales of dark Inotite. 
It contains from 55 to G5 i»er cent of silica, thus dittering from quartz-porphyiy and 
felsite in its .smaller proportion of this acid. It is also rather more easily scratched with 
the knife, but exce])t by chemical or microscopical analy.sis, it is often impossible to 
draw a distinction between this rock and its etpiivalents in the acid series. 

Orthoclase-iK)ii)hyry occurs in veins, dykes, and intrusive sheets. Probably many 
so-called *' felstoncs," whether occurrmg as lavas or as intrusive masses among the oldu* 
Pala'ozoic formations, are i-eally orthoclase-iwrphyries. Some highly micaceous varieties 
have l»een calle<l Mica -trap — a vague term under which have also been included 
Alinettcs, Micaceous Quartz -i)orphyries, &c. The name Lamprophyre, originally 
given by Ci!iiml»el to some mica-traps from the Fichtelgebirge, has been proposed by 
Kosenbusch as a general term for the Mica-traps, divisible into two groups — the Ortho- 
clastic, or syenitic. where the felspar is orthoclase (Miuettes). and the Plagioclastic or 



PAKT u § vii MA SSIVE ROCKS— DIORITE 1 6 5 

dioritic, where the felspar is a plagioclase variety (Kersantites ).^ The Umproph3rres 
occur abundantly as dykes or veins of a fine-grained texture, and dull reddish to 
brownish colour, among the older Palaeozoic rocks of Britain.^ 

The orthoclase-porphyry of Pieve in the Vicentin was found by Von Lasaulx to have 
the following composition : — silica, 61-07 ; alumina, 18'56 ; peroxide.s of iron and 
manganese, 2*60; potash, 6*83; soda, 3-18; lime, 2-86; magnesia, 1'18 ; carbonic 
acid, 1-36 ; loss, 2-13— specific gravity, 2-59.* 

Diorlte.^ — Under this name is comprehended a group of rocks, which, possessing a 
granitic structure, differ from the granit'CS in their much smaller percentage of silica, and 
from the syenites in containing plagioclase instead of orthoclase as their chief con- 
stituent. They are sometimes divided into two sections, the quartz-diorites and the 
nonnal diorites. Many of these rocks were formerly included in the general division of 
"Greenstones." 

Quartz-diorite — a holocrystalline mixture of plagioclase (oligoclase, less fi*equently 
labrmdorite) and quartz ^^-ith some hornblende, augite, or mica. It outwardly resembles 
grey granite, and, indeed, includes many so-called granites. Its silica ranges up to 67 
per cent. In normal Diorite, quartz is almost entirely absent; hornblende and 
black mica occur together in some varieties, while pyroxene characterises others. 
Under the microscope a thoroughly crystalline structure is seen, and among the pyroxene- 
dioritea the felspar and pyroxene are sometimes intergrowu in o])hitie aggregates. The 
average chemical composition of quartzless diorite is : silica, 54 ; alumina, 16-18 ; 
potash, 1 •5-2*5 ; soda, 2-3 ; lime, 6-7-5 ; magnesia, 6-0 ; oxides of iron and manganese, 
10-14 ; mean specific gravity, about 2-95. 

Among the varieties of diorite, the following may be mentioned. Corsite (from 
Corsica) — a granitoid mixture of greyish-white plagioclase, blackish-green hornblende, 
and some quartz, which have grouped themselves into globular aggregations with an 
internal radial and concentric structure (Orbicular diorite, Kugeldiorit,Napoleon- 
ite — Fig. 8). Tonal ite'(from Monte Tonale, Tyrol) — a variety containing quartz, horn- 
blende, and biotite in strongly contrasted colours. Ej»i(liorite — a name given to ancient 
rocks which have originally been pyroxenic eruptive masses, but, by mctamor})hism, have 
acquired a crystalline re-arrangement of their constituents, the pyroxene being changed 
into hornblende, often fibrous or actinolitic, the felsjiar becoming granular, and the whole 
rock having acquired a more or less distinct schistose structure. The dark intrusive sheets 
aattociated with the crystalline schists of the Scottish Highlands and the north of Ire- 
land are largely epidiorites. Some of these rocks are rjuartziferous, but many of them 
belong to the basic series (see p. 627). 

As the granites pass into fine grained quartz-] porphyries, and the syenites into conii»aot 
orthoclase -porphyries, so the diorites have their close -textured varieties, which are 



' The typical locality for these rocks is Kersanton in Brittany, where they are dark -green 
and remarkably durable. A singular vein of kersantite, 3 to 6^ feet broad, has been traced 
for nearly five miles in the Harz. Lossen, Zeitsch. Deutsch. Ge^tl. Oes. xxxii. (1880), p. 445. 
Jakrh, Preuu. Oeol. Landuanst. 1880. A. von Groddeck, ojk at. 1882. M. Koch, np. cit. 
1886. BarroiB, Assoc. Fmnfaise (1880), p. 561 ; Ann. Stn:. OM. Nortl, xiv. (1886), p. 31. 

* For an account of the Lamprophyres of the classical district of the Plauen8cher-Grun<l, 
•ee B. Don, TscAennak*s Mineral MiUheiL xi. (1889). 

* Zeitseh. Deutsch. Oeol. Ues. xxv. p. 320. 

* On diorite, its structure and geological relations, consult the memoir on Belgian 
platonic rocks by De la Vallee Poussiu and A. Renard, Mim. Acad. Royale Behj. 1876 ; 
Behrens, Neues Jahrb, Min. 1871, p. 460; Zirkel, 'Microscopical Petrog.' p. 83. J. A. 
Phillips, Q. J, Oeol. Soe. xxxii. p. 155, and xxxiv. p. 471 — two valuable papers in which 
the constitution of some of the ''greenstones" of the older geologists is clearly worked out. 
Many of these ancient rocks are there shown to be forms of doleritic lava, and the change 
of their original sngite into hornblende is traced. 



1 66 OEOGNOS Y book u 



comprised under the general tenn Aplianite, divisible into Quartz-aphanite and Korwal 
aphanite. The general characteristic of these rooks is that the constituent minenli 
become so minute as to disappear from the naked eye. They are dark hea^y close- 
grained masses. They merge into the basic diabases (j). 170). 

Trachyte^ — a term originally applied to modem volcanic rocks possessing > 
characteristic roughness {jpaxvi) under the finger, is now restricted to a compact, 
usually i»le, i)orphyritic, frer^uently cellular, rock, consisting essentially of sanidine, 
with more or less triclinic felspar, augite, hornblende, and biotite, sometimes with apatitf, 
and tiidymite. It Is distinguished from rhyolite, or quartz- trachyte, by the absence of 
free quartz, and by the smaller i»roi)oi*tion of %itreous or microlitic (micro-felsitic) ground- 
mass. The sanidine crystals present abundant steam-()or6s and glass-inclusions, as wdl 
as honiblende-microlites and magnetite. In some varieties, the ground-mass appeazi 
to be entirely com|)osed of microlites ; in others, minor degrees of devitrification can be 
traced, until the ground-mass passes into a glass (trachyte-glass, obsidian). The trachyten 
of Hungary have been grouped as Augite -trachyte^ Amphihoh-traehyte^ and Biatik' 
trachyk. Average comixisition of Trachyte: — silica, CO-0-64'0 ; alumina, 17*0; pro- 
toxide and (peroxide of iron, 6*0-8*0 ; magnesia, 1*0 : lime, f3*5 : soda, 4-0; potiiih, 
2* 0-2* 5. Average specific gravity, 2*65. 

Trachyte is an abundantly difiused lava of Tertiary and Post-tertiar}" date. It 
occm's in most of the volcanic districts of Euro[)e (Siebengebirge, Nassau, Transylvaniik 
Bay of Naples, Euganean Hills) ; in the Western Territories of tlie United States * ; iB 
Xcw Zealand. It also occurs among the Carboniferous lavas of Scotland. 

Do mite (so named from the Puy-de-I)dme) is a |N)rous loosely aggregated trachyte, 
liaving a microlitic gi'ound-mass, through which arc dLs])ersed tridymite, sanidine, 
nnich plagioclase, hornblende, magnetite, biotite, and specular iron. Soda-traohyte 
(Pantellerite) is a variety rich in oligoclasc, found in Pantelleria. 

Phonolite (Xejiheline- trachyte, Clinkstone)* — a term suggested by the metallic 
ringing sound emitted by the fresh comimct varieties when struck, is applied to • 
com^tact, grey or brown, (piartzless mixture of sanidine and nephelinc, with nosean, 
hauyne, leueite, pyroxene, hornblende, or mica. The rock is rather subject to decom- 
I)osition, hence its tissui'cs and cavities ai'e frequently filled with zeolites. An avenige 
specimen gave on analysis — silica, 57*7 ; ahmiina, 20*6 ; iK)tash, 6*0 ; soda, 7-0 ; lime, 
1*5 ; magnesia, 0*5 : oxides of iron and manganese, 3*5 ; loss by ignition, 3-2 percent 
The si)ecific gravity may be taken as about 2-58. Phonolite is sometimes found splitting 
into thin slabs wliich can be used for roofing purj^ses. Occasionally it assumes a por- 
]ihyritic texture from the pres«nici> of larg(» cr^'stals of sanidine or of honibleude. When 
the rock is ]iurtly decom{)Osed and takes a somewhat jtorous texture, it resembles nofnnal 
trachyte. 

It is a thoroughly volcanic rock, and generally of Tertiary date. It occurs some- 
times filling the pi]M?s of volcanic orifices, sometimes as sheets which have been ]ionred 

' Ou tracliyte, sec Zirkel, * Micro. Petrog.' p. 143. King in vol. i. of *E«xplor. 40th 
l*arallel,' }>. 578. Ou the relative age and classiticuiion of Hungarian trachytes, Siab6, 
Zeitsch. JJndsc/t. Oeol. Oes. xxix. p. 635, and *Compte rend. Cougres intemationale de 
Geologic' (1878), Paris, 1880. For the Scottish Carboniferous trachj'tes see Presidential 
Address to the Geological Society 1892, and F. H. Hatch, Trans. Roth Soc, Min. 18W. 

' It would apfiear that mivrh of what has been regarded as trachyte in Weatem Ameria 
is andesite, consisting essentially of plagioclase, and not of sanidine. The normal trachytes 
are now descril^ed as honibleude-mica-andesites, and the augite- trachytes are hypenthene- 
augite-andcsitcs, most of the rest l)eiDg dacites. and some of them rhyolites. Hague and 
Lldings, Amcr, Jointi. .Srt. xxvii. (1884), p. 456. 

^ Boricky, ' Petrograph. Stud. Phouolitgestein. Buhmeus.' — Archiv Landesdureh/onehung 
Bohvien, 1874. G. F. Fohr, " Die Phonolite des Hegau's," Vrrh. Phys. Med. Oes. Witnbury, 
xvui. (1883). F. H. Hatch, Ttvnfi. Hv;i. f^in-. Eilhi. 1892. 



PART u § >-ii MASSIVE ROCKS— AN DESITE 167 

out in the foim of laya-streams, and sometimes in dykes and veins, as in Bohemia and 
Anvergne. Some of the great bosses or eruptive vents connected with the trachyte 
lavas of the Garleton Hills, Haddingtonshire, have recently been determined by Dr. 
Hatch to be true phonolites. 

With the phonolites may be classed Leucite-trachyte, or Leucite-phonolite, 
where the felspathoid is leucite instead of nepheline, and Nosean- trachyte (Nosean- 
phonolite), or Hauyne- trachyte (Hanyne-phonolite), with nosean or hauyne taking 
the place of the felspar of ordinary phonolite. 

Andasita — a name originally given by Yon Buch to some lavas found in the Andes, 
is now applied to a large series of rocks distinguished from the trachytes in that their 
felspar is plagioclase, and i)assing by the addition of olivine into dolerite and basalt. 
In fresh examples they are dark grey, or even black rocks with a compact ground-mass, 
through which striatcKl felspar prisms may generally be observed. They often assume 
oellular and porphyritic structures. At the one end of the series stand rocks containing 
free silica (Dadte), while at the other are basalt-like masses of much more basic com- 
position (Augite-andesite). Under the microscope the ground-mass presents more or 
leas of a pale brownish glass with abundant felspar microlites. 

Dacite (Quartz-andesite) — composed mainly of plagioclase, quartz, and mica, with 
a varying amount of sanidine as an accessory constituent, and, by addition of hornblende 
and pyroxene, graduating into homblende-andesite. The ground-mass has a felsitic, 
sometimes spherulitic, glassy, or finely granular base. Composition : silica, 69*36 ; 
alumina, 16*23 ; iron oxides, 2*41 ; lime, 3*17 ; magnesia, 1*34 ; alkalies, 7*08 ; water, 
0*45. Mean specific gravity, 2*60. This rock is extensively developed in the Great 
Basin and other tracts of western North America among Tertiary and recent volcanic 
outbursts. 

Hornblende-andesite' consists of a trielinic felspar (usually oligoclase), with horn- 
blende, augite, or mica. The ground -mass resembles that of trachyte, presenting 
sometimes remains of a pale glass. The por])hyritic minerals frequently show evidence 
of having been much corroded before consolidation. Composition : silica, 61 '12 ; alumina, 
11-61 ; oxides of iron, 11*64 ; lime, 4-33 ; magnesia, 0-61 ; potash, 3*52 : soda, 3*85 ; 
ignition, 4*36. Homblende-andesite is a volcanic rock of Tertiary and post-Tertiary 
date found in Hungary, Transylvania, Siebengcbirge, and in some of the Western 
Territories of the United States. According to researches by ^Lessrs. Hague and 
Iddings, gradations from this rock into basalt and hypersthene-andesite can he traced 
in California, 'Oregon, and Washington. These rocks, therefore, cannot be said to have 
sharply defined and distinct fonns.^ Under the name of Hornblende- mica -andesite 
American petrographers have described a frequent variety of rock throughout the Great 
Basin, characterised by the vitreous appearance of its felspar, its rough porous trachyte- 
like ground-mass, and the presence of mica as an essential constituent. This term yn\\ 
include a large proportion of the rocks hitherto classed as trachyte.s, but in which the 
felspar proves to be plagioclase and not sanidine.' 

Pyroxene-andesite — consisting of labradorite or oligoclase, with augito (less 
frequently a rhombic pyroxene) and abundant magnetite, sometimes with hornblende 
or mica, forming a dark heavy basalt-like compound, with a com])act sometimes more 
or less distinctly vitreous ground-mass. Composition : silica, 57*15 ; alumina, 16*10 ; 
protoxide of iron, 13*0 ; lime, 5*75: magnesia, 2*21 ; potash, 1*81 ; soda, 3*88. Mean 
specific gravity, 2*75-2*85. 

It was formerly supjwsed that the pyroxene of the andesites was always augite. But 
rhomhic forms of the mineral have now been frequently detected. Under the name of 



^ See Ztrkel, *Micro8Cop. Petrog.' p. 122. King, in vol. i. of 'Explor. 40th Parallel,' 
p. 562. Hague and Iddings, Amer. Jovm. Sci. xxvi. (1883). p. 230. 
« Amer. Journ. Sci. Sept. 1883, p. 233. 
' Hague and Iddings, Anier. Journ. Sci. xxvii. (1884), p. 460. 



168 GEOGNOSY BOOKn 



Hypersthene-andesite, certain Tertiary or recent rocks, stretching oyer Tart 
Western America, have been described as associated with other andesites and faanltaL 
Tliey are black to grey, or reddish-grey, in colour, and vary in - texture from denM^ 
tlioroughly crystalline forms, to others approaching wliite glassy pumioe, the Ian 
under the microscope ranging from a brown glass to a holocrystalUne stracture. Tht 
magnesian silicate is pyroxene, chiefly in the orthorhombic form as hypersthene, bit 
partly also augit«. An analysis of the pumiceous form of the rock gave 62 per cent «f 
silica, while the |>ercentage of the same constituent in the glass of the base was found 
to rise to 69-94.» 

Pyroxene -andesite occurs in dykes, lava -streams, plateaux, sheets, and neck -like 
bosses in regions of extinct and active volcanoes, as in the Inner Hebrides, Antrim, 
Transylvania, Hungary, Santorin, Iceland, Teneriifc, the Western Territories of North 
America, the Andes, New Zealand, &c. Many of the rocks of these regions now classed 
under this name have long been known and described as dolerites and basalts. Indeed, 
there is the closest relation between them and the true olivine -bearing dolerites and 
basalts. The latter occur among the Tertiary volcanic plateaux of Britain, interstratified 
with rocks whicli, not containing oli%^e, have been placed among the andesitM. 
Neither in their mode of occurrence nor to the eye in hand specimens is there any good 
distinction to be drawn bet\veen them. Under the name of Tholeite some intensti^g 
augite-andesites have been described, in which the felspar prisms form a network fiUsd 
in with graimlar augite and interstitial matter (intersertal structure). In other varietki 
of andesite the felsi)ar-mesh has been filled with lai^e crystalline patches of angite^ 
which thus encloses the felspar (ophitic structure). 

Tephrite (Nepheline-andesite, Leucite- andesite, Nosean- or Hanyne- andesite)— a 
group of andesites, in which the felsiiar is ]>artly replaced by one of the felspatlioidl» 
nepheline, leucite, nosean, or hauync. 

Porphyrite — a name for old forms of andesite which have generally undeigaw 
considerable alteration, and conse(piently appear as dull, sometimes earthy, genenllj 
reddish or brownish rocks. When fresh they are dark grey or black. They an 
commonly iK)rphyritic, and show abundant scattered cr^'stals of plagioclase, Ims 
eonmionly of mica. Their texture varies from coarse crystalline to exceedingly dose- 
grained, passing occasicmally into vitreous varieties (Yetholm, Cheviot Hills). Bo(^ 
of this ty]>e have been abundantly }X)ured forth as lavas during Palteozoic time, and tfasj 
occur as iuterstratilied lava-lx^ds, eruptive sheets, dykes, veins, and irregular bosses. 
In Scotland they form masses, several thousand feet thick, enipted in the time of tbe 
Lower Old Red Sandstone, and others of wide extent and several hundred feet in 
depth 1>elonging to the Lower Carboniferous period. In (lermany porphyrites appear 
also At numerous points among formations of later Pabeozoic age. 

Prop y lite — a name given by Richthofen to certain Tertiary volcanio rocks of 
Hungar}', Transylvania, and the Western Territories of the United States, consisting of 
a triclinic felspar and hornblende in a fine-grained non-vitreous ground-mass, and elosdy 
relatc^d to the Ilomblende-andesites. Their distinguishing feature is the great alteration 
which they have undergone, whereby their ferro-magnesian constituents have been con- 
verted into chlorite, and their fels|>ars into epidote. Some quartziferons propylites 
have been described by Zirkel from Nevada, wherein the i[uartz abounds in liquid in- 
clusions containing briskly -moving bubbles, and sometimes double enclosures with an 
interior of liquid carbon-dioxide.- A s[)ecimen from Storm Canon, Fish Creek Mountains, 



^ Whitman Cross, B»U. U, S. Gtd, Survey, 1883, No. 1. Hague and Iddings, A 
Journ. Sci. xxvi. (1883), p. 226 ; xxvii. (1884), p. 457. 

- Zirkel's * Microscopical Petrography,* p. 110. King, " Exploration of 40th Puallel," 
vol. i. p. 545. C. E. Button's '' High Plateaux of UUh '' ( U.S. Geographical and CMoffiaA 
Survci/ of the Rocky Mountains), chaps, iii. and iv. Hague and Iddings, Amer, Jaum. ScL 
1883. 



PART n § vii MASSIVE ROCKS— GABBRO 169 



contained silica, 60*58; alumina, 17*52; ferric oxide, 2-77; ferrous oxide, 2-53; manganese, 
a trace ; lime, 3-78 ; magnesia, 2-76 ; soda, 3-30 ; potash, 4-46 ; carbonic acid, a trace. 
Lo68 by ignition, 2*25 ; specific gravity, 2- 6-2-7. The geologists of the Geological 
Survey of the United States believe that the rocks included under the term " propylite " 
in the western parts of America represent various stages of the decomposition of granular 
diorite, porphyritic diorite, diabase, quartz-porphyry, homblende.-andesite, and augite- 
andesite.^ The name has been more recently applied by Rosenbusch to rocks which 
have ondergone alteration by solfataric action. 

iii Basic Seriet. 

This third series of eruptive rocks is distinguished by its low silica percentage, and 
the relative abundance of its basic constituents. A similar range of structure can be 
traced in it as in the other two series. At the one extreme come rocks with a holo- 
cryvtalline structure like the gabbros. These pass into others of a hemi- crystalline 
character, where, amid abundant crystals, crystallites, and microlites, there are still 
traces of the original glass. At the other end lie true basic volcanic glasses, which 
externally might be mistaken for the pitchstones and obsidians of the acid rocks. 

Gmblnro ^ (Euphotide) — a group of coarsely crystalline rocks composed of plagioclase 
(labradorite) or anorthite, magnetite or titaniferous iron, and some ferro-magnesian 
mineral, which in the normal gabbros is augite or diallage, but may be a rhombic pyroxene, 
hornblende, olivine, or mica. These minerals occur in allotriomorphic forms, as in 
granite ; but they sometimes assume ophitic relations which lead into the rock termed 
dolerite. The felspar has often lost its vitreous lustre and passed into the dull opat^ue 
condition known as saussurite. Tlie augite is usually in the fonn of diallage, distin- 
guished by its schiller-spar lustre. 

Oabbro occurs as an eruptive rock among the older fonnations, likewise in large bosses 
and dykes in volcanic cores of Tertiary age (Mull, Skye). Average composition : silica, 
49; alumina, 15; lime, 9*5; magnesia, 9*7; oxides of iron and manganese, 11*5; 
potash, 0*3 ; soda, 2*5. Loss by ignition, 2*5 ; specific gravity, 2-85-3-10. 

The following varieties may be noticed: Olivine-gabbro — a granitoid or ophiti(^ 
oom)M>und of plagioclase, augite, olivine, and magnetic or titaniferous iron ; good exam])leH 
are found among the deep-seated parts of some of the Tertiary volcanic vents of the 
Inner Hebrides. Hy persthene-gabbro or Norite (Hyi)er8thenite, Hyperite, Schillcr- 
fels) — with a rhombic pyroxene in addition to or in place of the augite. Troctolite 
(Forellenstein) — a mixture of white anorthite with dark -green olivine, i-eceives its name 
from the supposed resemblance of its speckled appearance to that of the side of a trout. 
Pyroxene-granulite (granular diorite, trai>-granulite) — consisting of plagioclase, 
pyroxene (monoolinic and rhombic), hornblende, and garnet, distinguished by the 
granular condition of these minerals, and found among gneisses and other schistose 
rocks ; this is probably an altered condition of some original pyroxeuic eruptive rock. 

Dolsrite — an important group of basic rocks, which connect the gabbros with the 
basalts and include many of the rocks once termed *' Greenstones." They are composed 
of labradorite (or anorthite), with some ferro-magne.sian mineral (augite, enstatite, olivine, 
or mica) and magnetic or titaniferous iron. As a rule, they are holocrystalline, the 
constituent felspar and pyroxene or olivine l>eing characteristically grouped in ophitic 



* G. F. Becker on the Comstock Lode. Reports of U.S. Gedogiral Surrci/ 1880-81, and 
his fall memoir in vol iiL of the Monographs of U.S. OeoL Sun-ei/ (1882). Hague and 
Iddings, Amer. Joum, ScL xxvii. (1884), p. 454. 

' On Gabbro see Lessen, Z, Deutsche GeM. Ges. xix. p. 651. Lang, op. rif. xxxi. p. 
484. Zirkel on Oabbros of Scotland, op. cii. xxiii. 1871. Judd, Quart. Joum. GtAil. Soc. 
xlu. (1886), p. 49. G. H. Williams, BuU. U.S. Ged. Surv. No. 28 (1886). F. D. Cliester, 
op. eU. -No. 59 (1890). M. E. Wadsworth, Geol. Surv. Minnesota, Bull. 2, 1887. 



1 70 GEOGNOS Y book n 



structure, but a little residual glass may occasionally be detected. They occur in boaei, 
intrusive sheets, and dykes, esi)eeially as the subterranean aecompauiments of tW 
volcanic action which has tln'ouii out augite-andosites and liasalts to the snrlkce. 

Normal or oidinary dolerite consists of plagioclasc and augite, with magnetite « 
titanic iron and frequently olivine. Average composition: silica, 45-55; alnnuai, 
12-16 : lime, 7-13 ; magnesia, 3-9 ; oxides of iron and manganese. 9-18 ; potash, 0-1 ; 
soda, 2-5. Loss by ignition (water, &c.), 0*5-3 ; specific gra\'ity, 2*75-2-96. 

Diffei'ent names have l>een proposed for the chief varieties. The moat imiK>rtant «f 
these are Olivine-dolerite — a dark, heavy, close-grained finely -crystalline rock, witk 
scattered olivine, apt to weather with a brown ciiist. OH vine-free dolerite— • 
similar rock but containing no olivine. Enstatite-dolerite contains enstatite h 
addition to the other ingredients. Nepheline-dolerite, has the felspar lai^gfly cr 
entirely replaced by nepheline (see Nephelinite, p. 172). 

Ah varieties of dolerite depending for their peculiarities mainly upon their antiqnitf 
and the consequent alteration they have undergone, we may include the rocks oon* 
prehended under the term Diabase.* This name was given to certain dark green or 
black eruptive ro(;ks found in older geological formations, and consisting essentially of 
triclinic fels[>ar. augite, magnetite or titaniferous iron, apatite, sometimes oliviiK^ 
usually with more or less of diffused greenish chloritic substances (\iridite) which hiw 
resulted from the alteration of the augite or olivine. Tlie average com|>08ition of typkil 
diabase may be taken to be : silica, 48-50 ; alumina, 16*0 ; protoxide of iron, 12-15; 
lime, 5-11 ; magnesia, 4-6 ; potash, 0.8-l'5 ; soda, 3-4*5 ; water, l«5-2. Speeifie 
gravity about 2*9. There Is generally carbonic acid present, united with some of the 
lime as a decom|K)sition ])roduct. As in ordinary dolerite, gradations may he tneed 
from coarsely crystalline diabase^ into exceedingly fine-grained and compact Tarietki 
(Diabase -aphanite), which sometimes assume a fissile character (Diabase-schiefer) what 
they have been subjected to crushing or cleavage. Some kinds present a porphyritic 
structure, and show dis^iersed ciystals of the conqionent minerals (Diahase-porphyiji 
Labrador-iwrphyry, Augite- i)orphyry) ; or, as in some varieties of diorite, a concretioiiiiy 
arrangement Ls ])roduced by the appearance of abundant {)ea-like bodies of a compact felritie 
mat(;rial, imbedded in a compact or finely crystalline ground-mass (Variolite). Whei 
the green com|)act ground-mass contains small kernels of carbonate of lime, sometinM 
in great numbers, it is calleil Calcareous aphunite or Calcaphanite. Sometimes the roi^ 
is abundantly amygdaloidal. Though, as a nile, free silica does not occur in it, sone 
varieties found to contain this mineral, i)ossibly a secondary product, have been 
distinguished as Quartz -diabase. The i)resence of olivine has suggested the name 
01ivine-dial>ase as distinguished from the normal kinds in which this mineral is absent 
A variety containing hornblende is termed Proterobase. 0[>hite, a variety occnrring in 
the Pyrenees, contains diallagc and epidote (see p. 120). 

Diabase occui-s both in contemporaneous beds and in intnisive dykes and sheets. 

Basalt ^ — a black, extnmiely conqiact, a]>)varently homogeneous rock, which breaks with 
a splintery or conchoidal fracture, and in which the component minerals can only be 



* Tlie student will find in the Zcitschnft. Ihutsch. Geol. (fes. 1874. p. 1, an importsBt 
memoir by Dathe on the composition and structure of dial>ase. See also Zix1nl*i 
* Microscop. Petrog." p. 97. 

* Michel-Levy, BuH. Son. O^yf. Francey 3rd ser. xi. p. 282. Geikie, Trana, Bo^, Soc 
Edin, xxix. p. 487. 

^ On basalt rocks see Zirkel's ' Basaltgesteine,' 1870. Boricky's ' FetrographisGhe 
Studien en deu Ba.saltgesteiueu Buhmeus,* in Archiv fiir ytUvnciss. Lan4Uadurdi/9r- 
schuuff von Biihmeny ii. 1873. All|>ort, Q. J. Geol.\ A'in: xxx. p. 529. Oeikie, 2V«iu. 
Rot/. Site, EtUiu xxix. Mohl, Xoi: Act. AcAui. Lenp. i\irol. xxxvi. (1873), p. 74 ; iVcMO 
JaJirb. 1873, pp. 449, 824. F. Eiohstailt on Basalts of St^auia. Sreriges Geoi, VnderwA^ 
ser. c. No. 51, 1882. K. Svedmark, op. cit. No. 60, 1883. 




FART II § Tii MASSIVE BOCKS— BASALT 171 

obaerred with the microscope, unless where they are scattered porplijritioally through 

the mass (Fig. S2). The minerals coiiaist of plagioclase (tabradorite or anortbite), 

pjrroxene (usnallf augite, but i>ccasiaual!y a rhombic form), olivine, mifrnetite or 

tituiiferoDS iroD. Many yea« ago, Andrews 

detected native iron in the basalt of Antrim, and 

more recently Nordenskibld found thia substance 

abundantly difliued in the basalt of Disco Island, 

occurring even in large blocks like meteorites 

(aiUV, p. 68). The gronnd-mass of basalt pre- 

seats under tbe microscope traces of glass in 

which are imbedded minute granules, hairs, 

needles, and microlites of felspar and sugile. 

The proportion of this base varies within wide 

limit*, insoinucb that while iti some jiarts of a 

banlt it so preponderates that the individual 

ciTitala are scattered widely through it, or are 

drawn out into beautiful streaks and eddies of 

Bnxion stnicturo, in others it almost disap|>ears. 

and the rock then appears as a nearly crystalline I 

maaa, which thus graduates into dolerite and (nugnlMed^ Th* large sbsded crystal- 

basic andesite. The component minerals fre- \^^ uI^^'ZmwUU pti^^^n'st. 

quently appear porphj-ritically dispersed, espe- gUMlaw. A few Aiiglte prbims occur 

eiallj the olivine, the pale yellow grains of which, lo the right of the centre of the 

which are characteristic, drawing, are iggn^ati'd into b large com- 

Two types of basalt have been recognise.! J^nttf^""'" '^' '''"'' *"""'" '" 
in the great basaltic outbursts of Western '"' 

America: (1) the porjihyritic, consisting of a glassy and microlitic or micro- crystal- 
line groand-maas, bearing relatively large crystals of olivine, felspar, and occasion- 
ally augite, a structure showing close relations to that of many andesites : (2) the 
granular (in tbe sense in which that tenn is used liy Rosenbnsch, oiite, p. US) — an 
aggregate of quite uniform grains, com|iosed of Kell-devcloj>ed plagioclase and olivine 
crystala, with ill^delined patches of augite, and frequently with a considerable amount 
of^aBa-baae. By diminution of olivine and augmentation of silica, and the apimarancc 
of hypersthene, gradations can lie traced from true olivinc-liasalta into normal andesites. 
Basalts with free qnsrtz are not infreijueiit in Western America.' 

Basalt occurs in amorphous and columnar sheets, which may alternate with each 
other or withMSOciated tuRs. It also forms abundant dykes, veini, and intrusive bosses. 
It frequently assumes a cellular structure, which becomes aniygdaloidal by the dejiosit 
of cahnte, leoUtes, or other minerals in the vesicles. A relation may be traced 
between the development of amygdates and the state of the rock ; the more amygdaloidal 
the tock, the more is it decomposed, showing that the aniygdales have probably in 
large measure been derived by infiltrating water from the basalt itself. 

Vitreous Basalt (Baaalt-gUss, Tachylyte, Hyalonielan).'— Basalt jMsses into a 
condition which, even to the naked eye, is recognisable o-s that of a true glass. This 
more especially takee place along the edges of dykes and intrusive sheets. Where an 
eztemal skin of the original molten rock has rapidly cooled and consolidated, in contact 
with the rocks through which the eniption took place, a transition can l>e traced 
within the space of less than a <[narter of an inch from a crystalline dolerite, anamesite, 
baaalt, or andesite into a black glass, which under the microscojie assumes a jiule brawn 

■ Hague and Iddiogs, Amer. Jotirn. &i. ixvii. (1884), p. *S8. hidings, "/J. til. xxxvi. 
<ieS8), p. a08, Bull. U. S. aeU. S-ff. No». 66 and 7» (J. 9. Diller). 

■ See Judd k Cole, Q. J. Ocol. S'k. ixiii. (1883), p. 444. Cole, op. cU. >liv. (I8SS), p. 
300. Cohen, Jrnuit Jakrb. 1876, p. T44 : ISSO (vol. ii.), p. 2Z (Saudwich Islands). 



172 GEOGNOSY Boocn 



or yellowish colour, and is isotropic, but generally contains abundant microlites, 
times with a globular or aphenilitic concretionary structure. In such cases it 
disputable that this glass represents what was the general condition of the whole molten 
mass at the time of eruption, and that the present crystalline structure of the rook wu 
dcyelo|)cd during cooling and consolidation. The glassy forms of basalt undergo altcn* 
tion into a yellowish substance called Pa 1 agon i t e (p. 1 38). It is worthy of remark tfait 
in the analyses of vitreous basalts, the jtercentage of silica rises usually above, whik 
their spccitic gravity falls below, that of ordinary crystalline basalt. 

Tlie average composition of basalt is — silica, 45-55 : alumina, I0-] 8 ; lime, 7-14 : 
magnesia, 3-10 ; oxides of iron and maganese, 9-16 ; iK)ta8h, 0'5-3 ; soda, 2-5. Lm 
by ignition (water, &c.), 1-5 ; specific gravity, 2 '85-3 '10. 

The basalt-rocks are thoroughly volcanic in origin, a])i>earing in lava-streams, plateanXi 
sills, necks, dykes, and veins. The columnar structure is so common among the finer- 
grained varieties that the term '* basaltic " has been popularly used to denote it. M 
already stated, it has been assumed by some writers that luisalt did not begin to be 
erupted until the Tertiary period. But true basalt occurs abundantly in Scotland ai a 
product of Lower Carboniferous volcanoes, and exhibits there a variety of types rf 
minute structure.* 

Basic Pumice. — Though the acid lavas furnish most of the pumice with which «• 
are familiar, some of the basic kinds also assume a similar structure. Thus at Hawui, 
the basic, pyroxenic or olivine lavas give rise to a pumiceous froth. 

Melaphyre — a name originally proposed by Brongniart and subsequently applied ii 
various senses by different writers to include rocks which range in structure and ecas- 
])osition from the more basic andesites to true olivine-basalts. Tlie melaphyrei ftr 
the moat part l>elong to pre-Tertiary eruptions (though some Tertiary lavms ham 
been described as melaphyre) and have undergone more or less alteration. If the word 
is to be retained as a definite rock-name it should be restricted to an altered typo, ask 
now generally agreed, and preferentially to the older altered basalts. The melaphyres «3Di 
then bear somewhat the same relation to the basalts that the diabases do to the doleiitn 
and the i)orphyrites to the andesites. But it must necessarily happen that difficulty will 
be experienced in deciding which of the three names would be best applied to some oftbi 
eruptive rocks of the older geological formations. The melaphyres, as thus defined, tit 
somewhat dull, dark brown, reddish, or green rocks, often amygdaloidal and showing 
their iK)rphyritic minerals in an altered condition, the olivines especially being changBd 
into seri>entine or replaced by magnetite or even by htematite.^ 

Nepheline-basalt (Nepheline-Basanite). — Zirkel proved that certain black heavy 
rocks, having externally the aspect of ordinary basalt, contain little or no felsptf, 
the i»art of that mineral being taken in some by nepheline, in others by leucite.' Tbtf 
are volcanic masses of late Tertiary age, but occur much more sparingly than the txns 
basalts. They are found in the Odenwald, Thuringer Wald, Erzgebiige, Baden, ftc. 
Mean composition — silica, 45-52; alumina, 16'50 ; ferric and ferrous oxides, 11*30; 
lime, 10-62; magnesia, 4-35; }K>tash, 1-95; soda, 5-40; water, 2-68. Mean speeifie 
gravity, 2-9-3-1. Xephelinite is a fonn of basalt \\'ith no felspar or olivine. 

Leucite-basalt (Leucite-Basanite) contains little or no felspar, but has leuoite in 
place of it. Externally it resembles ordinary basalt. This rock occurs among the 



^ See Trans. Royal. Soc. Edin. xxix. (1879), p. 437, and Presidential Address, QMort 
Joitrn. Oeol. Soc. (1892), p. 129, where the types of microscopic structure observed by Dr. 
Hatch are enumerated. 

' For some account of the use of the word melaphyre see Brongniart, ' Cla8sificatio& ft 
Caracteres mineralogiques des Roches homogenes et heterogenes,' 1827, p. 100. Nannuum, 
* Lehrbuch der Geognosie,' i. p. 587. Zirkel, *Petrographie,* ii. p. 39. Roaenhnsdit 
' Mikroskop. Physiogr.' ii. p. 484. 

3 * Basaltgesteine.' 1870. 



PART n § N-ii MASSIVE ROCKS—SERPENTINE 173 



extinct volcanoes of the Eifel and of Central Italy, and forms the lavas of Vesuvius. 
Leucitite contains no felspar and no olivine. 

Melilite-Basalt. — In continuation of Zirkel's research, A. Stelzner has shown that 
in some basalts the part of felspar and nepheline is played by melilite.^ In outer 
appearance the rocks possessing this com[>osition, and to which the name of Melilite-basalt 
lias been given, cannot be distinguished from ordinary basalt. Under the microscoi)e, the 
ground-mass ap|)ear8 to be mainly composed of transparent sections of melilite, either 
disposed without order, or ranged in fluxion lines round the large olivine and augitc 
crystals ; but it also contains chromite (t), microlitic augite, brown mica, abundant 
magnetite, with perowskite, a^Mitite, and probably nepheline. (Swabian Alb, Bohemia, 
Saxon Switzerland, kc.) 

Under the awkward name of " ultra-basic," the following group of rocks is included 
in which the proportion of silica sinks to a still smaller amount than in the basalts. 

Umlmzgite (Magma- basalt)— a iine-graine<l to vitreous rock composed of augite, 
olivine, magnetite or titaniferous iron, and apatite. The base is generally glassy and the 
proportion of silica in the rock is only about 42 per cent. The ty])ical localit}' is Lim- 
burg, near the Kaiserstuhl in Baden. 

Paridotite Group. — The rocks here embraced, stand at the extreme end of the basic 
igneous rocks as the rhyolites and granites stand at the op]K)site end of the acid series. 
They contain no fels])ar, or at least an insigniticaut projiortiou of it, and consist of 
olivine, with augite, hornblende or mica, magnetic or titaniferous iron, chromite and 
other allied minerals of the spinel type. Tliey contain — silica, 39-45 ; alumina, 0-6 : 
ferrous oxide, 8-10 ; lime, 0-2 ; magnesia, 35-48 ; and have a mean specific gravity 
between 3*0 and 3 '8. When quite fresh these rocks have a holocr}'stalline structure, 
but they are generally more or less altered, and in their extreme condition of altera- 
tion fonn rocks known as ser[)entines. They occur for the most ]»art as intmsive masses 
belonging to the deejier-seated portions of volcanic eniptions. The following vaiieties 
may be noticed : — 

Pikrite * (Palaeopikrite, Pikrite-jKirphyry) — a rock rich in olivine, iisuall}- more or 
less serpen tinized, with augite, magnetite, or ilmenite, brown biotite, hornl)lende, or 
apatite ; occurs as an eniptive rock among Palaeozoic formations ; is closel}' related to the 
diabases into which by the addition of plagioclase it naturally i>asses. When horn- 
blende predominates over pyroxene the rock has been called hornblende- pikrite. 

Lherzolite' — so name<l from L'herz in the Ariege, is a holocrj'stalline rock com- 
posed of olivine, diallage, and a rhombic pyroxene, with a lesser proportion of a spinel- 
loid sometimes brown (chromite, picotite), sometimes gi*een (pleonast), and iron ores. 

Dunite, named by F. von Hochstetter from the Dun Mountain, New Zealand, con- 
sists of a granitoid mixture of olivine with chromite or other spinelloid. Such a rock 
naturally by alteration into a serpentine. 

Saiptntiiia.^ — Under this name arc included rocks which, whatever may have been 



1 Seues Jahrb. (BeiUgeband), 1883, p. 369-439. 

' So named from wixp^j bitter, in allu.sion to the large pro|K)rtion of bitter-earth (Mag- 
character shared by all the peridotites. G umbel, ' Die Palaeolithischen Eruptiv- 
gtsteine des Fichtelgeblrges ' : Munich, 1874. 

* On the eruptive nature of Lherzolite, see A. Lacrois, Compf. rend. cxv. (1892), pp. 974, 
and 976. 

* See Tscheniiak, SUz, Akad. H'tV//, Ivi. July, 1867 ; it wa.s this author who lirsl 
showed the derivation of serpentine from original olivine rocks ; Bounev, Q. J. GV/V. :Sik\ 
xxxiii. p. 884, xxxiv. p. 769 ; OeoL Jiaff, (2) vi. p. 362 ; (3) i. p. 406 ; Michel-Levy, BuN. 
Sk. OM. France, vi. 3rd ser. p. 156 ; Sterry Hunt, Tronti. Roif, .Sor. f'anfufo, i. (1883) ; 
Datbe, Semes Jahrb, 1876, pp. 236, 337, where Garnet-serpentine and Bronzite -serpentine 
ai« described from the Saxon granulite region. J. S. Diller, Bt'l/. ('. >*. ('*•"/. .sVr/-. No. 38 
(1887) ; M. E. Wsdsworth, * Uthological Studies,' (1884), p. 118. 



174 



GEOGNOSY 



BOOKO 



their □riKJiial character and composition, now cousist mainly or whoUjr of . 
Ab already stated, olivine readily jiassea into the condition of serpentiDe, whila tbe otka 
miiieraU may rciuain nearly unaflwt«d, aa in admirably seen in aome pikrite& IM 
serpentine- rocks originally ton aiateil principally of olivine (see Fig. 38.) DiOTitfi,pUn^ 



Pij|. sa.— SUhi;^ 




ttw altentlan talf a 



and oilier i-ouks, consisting largely ofmagnesianHiticatea. likewise pass in 

varieties due to different jiliosex of alteration were jndged worthy of separate dt 

each member of the jieridotites might of course have a conceivable or actual n 

tive among tlie seriwn tines. But without atteniptiug this minutciieaa of oloaiifintiai, ' 

we may with advantage treat by itaelf, as deierving special notice, the maMiTa &(■ 

of the mineral ser]icntiiie from whatsoever rock it way have originated. 

Massive Herpentine Ih a compact or finely granular, faintly glimmering, or doll nxl^ " 
easily cutorscratchediliuving a pre vailingdirty-greencolour.Bome times TariooslyitniU 
or flecked Hith brown, yellow, or red. It frefjuently contains other minenli banda 
seq)entine. One of ita commonest accoui|iaiiiments is chrysotile ur Hbrona Mrp«atii% 
which in veinings of a silky lustre oltcn ramiht's through the rock in all diractii»i 
Other common enclosures are bronzite, enstatite, magnetite, and chrome-BjqneU^ bMita 
traces of the original olivine, pyroxene, amphibole, mica, or felsjiar in the rooks vUifc 
have been altered into serpentine. 

Serpentine occurs in two distinct forms ; 1st, in Wis or bauds intercAlatod ainilBg 
schistose rucks, and associated esjiecially with crystalline limestones ; 2ndlf, in djfca^ 
veins, or liosnes traversing other rocks. 

As tu its mode of origin, there can iie no doubt that in most cases it was original^ 
an eruptive rock, as is dearly shown by ita aMD^ 
rence in dykes and irregular boaMa. Tie ft*- 
quent occurrence of reeognisahle olivine cxTital^ 
or of their still remaining contoura, in the midrt 
•if the eerfientine -matrix, affords good grounda fir 
assigning an erujitive origin to many letpMitiBea 
which have no distinctly eruptive estomal tbiM 
(Fig. 34). The rock cannot, of conna, luv« bMB 
ejected as the hydrous magnedaa silioato Wt- 
|)entine ; we must regard it aa hafing bMB 
originally an eru]>tive olivine rock, or ■ liigh^ 
hornblendic or micaceous diorite, or oliriM- 
gabbro. But, on the other hand, the interali- 
tion of beds of serpentine among achiatoaa rocki, 
and particularly the frequent occurrenoe of aa^ 
[lentine in connection with more or le^ altand 
limestones (West of Ireland, Highlands of Scotland) suggests aaothar mods of 




ie (30 DiBUictcn). 



FART II § vii SCHISTO:<E CRYSTALLINE ROCKS 176 



origin in theMe cases. Some writers have contended that such ser]>entine8 are pro- 
ductM of the alteration of dolomite, the magnesia having been taken up by silica, 
leaving the carbonate of lime behind as beds of limestone. Others have supi)osed the 
original rocks, from which the seri)entines were derived, to have 1)een a deposit from 
oceanic water, as has been suggested by Sterry Hunt in the case of those associated with 
the crystalline schists.^ Beds of serpentine intercalated with limestone might conceiv- 
ably have been due to the elimination of magnesian silicates from sea- water by organic 
agency, like the glauconite now found filling the chambers oi foraminifcra^ the cavities 
of corals, the canals in shells, sea-urchin spines and other organisms on the floor of 
the present aea.' Among the limestone and crystalline schists of Banffshire 
(pw 183), serpenti'ie occurs in thick lenticular l)eds which possess a schistose 
crumpled structure and agree in dip with the surrounding rocks. They may have lieen 
de|»0Hit8 of contemporaneous origin with the limestones and schists among which they 
occur, and in association with which they have undergone the characteristic schistose 
puckering and crumpling. Sometimes they suggest a source from the alteration of 
highly basic volcanic tuffs. In other cases they may have l)een erupted peridotites 
which have acquired a schistose character from the same jirocess of mechanical defonna- 
tion that has played so large a part in producing the foliation of the crystalline schists. 

III. Schistose (Metamorphic). 

In this section is comprised a series of rocks which present a re- 
markable system of divisional planes that are not original but have been 
superinduced upon them. At the one end stand rocks which are unmis- 
takably of sedimentary origin, for their original bedding can often be dis- 
tinctly seen, and they also contain organic remains similar to those found 
in ordinary unaltered sedimentary strata. At the other end come 
coarsely crystalline masses, which in many respects resemble granite, and the 
original character of which is not obvious. An apjMirently unbroken 
gradation can be traced between these extremes, and the whole series has 
been termed " metamorphic " from the changed form in which its mem])er8 
are believed now to appear. In the earlier stages the change has taken 
the form of cleavage as in ordinary slate. Even in slate, however, as 
already remarked (p. 134), a beginning may be detected in the development 
of crystalline particles, and the crystalline re-arrangement may be traced 
in constantly advancing progression until the whole mass has become 
crystalline, and forms what is known as a schist. 

The Crystalline Schists, properly so called, constitute a well-defined 
series of rocks. They are mainly comix)sed of silicates. Their structure 
is crystalline, but is distinguished from that of the Massive or Eruptive 
rocks by its more or less closely imrallel layers or folia, consisting of 
materisJs which have assumed a crystalline character along these layers. 
The folia may be composed of only one mineral, but usually consist of 
two or more, which occur either in distinct, often alternate lamime, or 
intermingled in the same layer. This structure resembles that of the 
stratified rocks, but it is differentiated (1) by a prevalent striking want of 
continuity in the folia, which, as a rule, are consi)icuously lenticular, 

1 « Chemical Bways,' p. 123. 

* According to Beithier, one of the glanconitic deposits in a Tertiary limestone is a true 
lerptntaiic. See Starry Hunt, ' Chem. Essays,* p. 303. 



176 aEOGNUSY a 

thickening out and then dying away, and reappearing after an inteml 
on the same or a ditferent plane (Fig. 35) ; (2) by a peculiar and vey 
characteristic welding of the folia into each other, the crystalline pu-tidii 
of one layer being so intermingled with those of the layers above ud 
below it thiit the whole coheres as a tough, not easily iissile masB ; (S) \ij 
a frequent remarkable and eminently distinctive puckering or crumpling 
(with frequent minute faulting) of the folia, which becomes sometimes ■ 




tolla, nttunl ikK- 



title as to l>e dieuernible only under the microscope^ (Fig. 37), hat ii 
often present conspicuouslj' in hand-specimena (Fig. 36), and can be tiaced 
iti increasing dimensions, till it connects itself ivith gigantic ctirvatnmof 
the Htrata, which embrace whole mountains. These characters are nS- 
cient to indicate a great difference between schistose rocks and oidiuiT 
stratified formations, in which the strata lie in continuous flat, panlH 
and more or less easily separable layers. ] 

In some instances, the folia can be seen to coincide with original bed- | 
<ling, as where a band of ijuartzito or of conglomerate is intercalated 
Ijetwecn sheets of phyllite or niita-schist. In such cases, there cannot b* 

> On the ininrosca|>ic xtnicture of llie crystnlliue schials nee Zirkel, ' Hiera«ooi<al 
Pctrograiili J ' (vol. vi. of KinK'a Ejploralioii of 40lli Parallel), 1876, p. 14. Allpoit, ^'. 
Gtnl. ,**«■. xxiii. |i. 407. Surliy, n/i. cil. sxivi. p. 81. Lehniann's ' Untcnuchnogtn ita 



PART II § v: 



SCHISTOSE CRYtiTALLlNE BOCKS 



any doubt that the rock, though now more or less re-constructed and crystal- 
line, was originally mere accumulated mechanical sediment Many clay- 
slates, phyllitea, and mica-schists are obviously only altered marine clays, 
and some of them still retain their recognisable fossils. From such rocks, 
gradations can be followed into chiastolitc-schist, mica-schist, and fine 
gneiss. Quartzites and quartz-schists often still retain the false-bedding 
of the original sandy sediment of which they are composed. The pebbly 




and conj^lomeratic bands associated with some schists afford convincing 
proof of their original clastic nature. Thus, at the one end of the schistose 
series we find rocks in which an original sedimentary character remains 
nninistakable. At the other end, after many intermediate stages, we 
encounter thoroughly amorphous crystalline masses, that bear the closest 
resemblance to eruptive rocks into which they insensibly pass. In such 
instances, there can be little doubt that the amorphous structure is the 
original one, which has become schistose by subsequent deformation 
(Book IV. Part VIII.) The banded arrangement of many coarse gneisses, 
however, may be an original segregation -structure, like that obsci'vable in 
sills and bosses of eruptive rocks (p. 613). 

In the more thoroughly rc-conslrncted antl re -crystallized schists 
all trace of the original structures has l«;en lost. The foliation is not 
coincident with bedding, nor with any structure of eruptive rocks, but has 



1 7 8 (rEOGXUS y iKHfE.li 



l)eeii detcraiinod by planes of cleavage or of shearing, or by the align- 
ment assumed by minends crystallizing under the influence of intense 
pressure. Along these surfaces the constituents have rearranged them- 
selves, and new chemical and mineralogical combinations have 1>e«n effected 
during the progress of the " metamoii)hisni." 

A rock ])ossessing a crystalline arrangement into separate folia is in 
English termed a Schist.^ This word, though employed as a general 
designation to describe the structure of all truly foliated rocks, is also 
made use of as a suffix to the names of the minerals of which some of the 
foliated rocks largely consist. Thus we have "mica-schist," "ehlorite- 
schist,'' " hornblende -schist." If the mass loses its fissile tendencj, 
owing to the felting together of the com^wnent mineml into a tough 
coherent whole, the word rock is usually substituted for schist, as in 
** hornblende-rock," **actinolite-rock," and so on. The student must bear 
in mind that while the possession of a foliated structure is the distiuctive 
character of the crystalline schists, it is not always present in every 
individual ]>ed or mass associated with these rocks. Yet the non-schistow 
portions are so obviously integi'al parts of the schistose series that they 
cannot, without great violation of natural affinities, be separated from 
them. Hence in the following enumeration they are included as common 
accompaniments of the schists. Quartzite also may Ije placed in this sab- 
diAision, though in its typical condition it shows no schistose structure. 

The origin of the crystalline schists has been the subject of long dis- 
cussion among geologists. AVerner held that, like other rocks of high 
antiquity, they were chemical precipitiites from a univei'sal ocean. 
Hutton and his followers maintiuned that they were mechanical aqueons 
sediments altered by subterranean heat. These two doctrines in various 
modifications are still maintained by opposite sch(X)ls. In recent jean 
much light has been thrown upon the oi-igin of the schistose structure, which 
has been shown to be in many cases due to the mechanical crushing and 
chemical re-adjustment and re-crystallization of the materials of both 
sedimentary and igneous rocks. This subject is discussed in a later part 
of this volume. (See Book IV. Part A'lll.) 

It is obvious that a wide series of rocks embracing variously altered 
forms of both sedimentary and igneous materials hardly admits of any 
simple syst(;m of classification. Kegarding them from the j)oint of view 
of the nature of the metamorphism they have undergone, geologists have 
sometimes grouped these rocks as resulting either from contact meta- 
morphism, that is, from the effects of the protrusion of igneous matter 
from within the earth's interior, or from regional metamorphism where the 
changes have been brought about by some widespread terrestrial disturb- 
ance (Book lY. Part YIII.) But this arrangement, though of value in 
discussing questions of metamorphism, has the disadvantage of introducing 
theoretical considerations, and of placing in different groups rocks which 

^ III French this term has no Micli (lufiuilo signitication, l)eing applied both to scliistt 
and to shales. In Oernian also tho corrv-.spondinjj word '* schicfer " designates jichista, bnt 
is also employed for non-rrystalline shaly rocks ; thonschiefer = clay-slate : schieferthon = 
nhah'. 



PART II § vii SCHISTOSE CRYSTALLINE ROCKS 179 



undoubtedly present the same general petrographical characters. Avoid- 
ing all disputed questions as to modes of origin, I shall group the schists 
according to their mineral characters, beginning with those which are 
obviously only a further stage of the alteration of clay-slates, and ending 
with the gneisses, which bear a close affinity to granites. 

1. Argillites, argillaceous schists, Phyllites.— The rocks included in this 
groux) may often be traced into the clay-slates described on p. 134. They mark a 
further stage of metamorphism, wherein besides mechanical deformation there has been 
m more or less decided re-crystallization of the materials, which is demonstrated by the 
abundant secondary mica and by the api)earance of such minerals as chiastolite, 
andalnsite, staurolite, garnet, &c. When a clay-slate becomes lustrous by the develop- 
m«ut of mica, it is known as Phyllite— a term which may be regarded as embracing 
the intermediate group of rocks between normal clay-slates and true mica-schists. 

Chiastolite- slate (schistemaole), a clay-slate in which crystals of chiastolite have 
been developed, even sometimes side by side witli still distinctly preserved graptolites 
or other organic remains ^ (Skiddaw, Aberdeenshire, Brittany, the Pyrenees, Saxony, 
Norway, Massachusetts, &c) Staurolito-slate, amicaceous clay-slate with crystals of 
staurolite (Banffshire, Pyrenees). Ottrelite-slate, a clay -slate marked by minute, six- 
aided, greyish or blackish green lamella; of ottrelitc (Ardennes, where it is said to con- 
tain remains of trilobites, Bavaria, Xew England). Di pyre -slate is full of small 
crystals of dipyre. Sericite-phyllite is a name proposed by Lossen for those com- 
pact, greenish, reddish, or violet sericite-schists in which the naked eye can no longer 
distinguish the component minerals. Mica-phyllite {phyllade gris feuilUU of Du- 
mont), a silky, usually very fissile slate, with minute scales of mica. German petro- 
graphers have distinguished by name some other varieties found in metamorphic areas 
and characterised by different kinds of concretions, but to which no special designations 
have been given in English. Knotenschicfer (Knotted schist) contains little knots 
or concretions of a dark -green or brown, fine-granular, faintly glimmering substance, of 
a taloose or micaceous nature, imbedded in a finely- laminated matrix of a talc-like or 
mica-like mineral.^ These aggregations appear to be in many cases incipient stages in 
the formation of definite crystals of sucli minerals as andalusite. In Fruchtschiefer 
the concretions are like grains of corn ; in Garbenschiefer, like caraway seeds; in 
Fleck schiefer, like Hecks or spots. Some of these rocks might be included with the 
mica-schists, into varieties of which they seem to jxass. Round some of the eruptive 
diabase of the Harz, the clay-slates have been altered into various crystalline masses 
to which names have been attached. Thus Spilosite is a greenish, schistose rock, 
composed of finely granular or comi>act felsj)athic material, with small chlorite con- 
cretions or scales. Desmosite is a schistose mass in which similar materials are dis- 
posed in more distinct alternations.' 

2. Quartz rocks.* — Quarti-Bchist (schistose (juartzite), an aggregate of gi-anular (or 



' A good illustration of this association is figured by Kjerulf in his ' Geologic des Slid* 
lichen und Mittleren Norwegen,* Plate xiv. fig. 246. See also Brogger's memoir on Upi>er 
Silurian fossils among the crystalline rocks of Bergen. Christiania, 1882. A similar 
association occurs in the graptolite-shales next the granite of Galloway, Scotland. 

* A. von Lasaulx, Ne^tes Jahrh. 1872, j). 840. K. A. Lossen, Z. Deutsch. Oeol. Ges. 
1867, p. 585 (where a detailed description of the Taunus phyllites will be found), 1872, p. 757. 

* Other names are Bandachiefer, Omtactschiefer^ &c. See K. A. lessen. Zeifsch. 
iPeutscK Oeo. Ges, xix. (1867), p. 509, xxi. p. 291, xxiv. p. 701. Kayser, op. cit. xxii. 

p. 103. 

* J. Macculloch, Trans. Oeol. Sik. 1st ser. ii. (1814), p. 450, iv. (1817), p. 264 ; 2nd 
ser. L (1819), p. 63. Loesen, Zeitsch. iMulsdi. Geol. Ges. xix. (1867), pp. 615-634. 



iS> 



'iKO'iXDsy, 



^ouuliti<:)i]UBrUwitha;<uffii'ielitcl<;veloli]nviitot tine fuliaof iiiicatoimiicrtamoreOTlM 
ilefiuLtelysoliiatosuHtructiire to tlie iw;k. 'Jlic (lUaii]«&raiire uf the mim give* quartab, 
und the greater prouiineiicc uf tliLs niiuovul slfonlij grodutious into mica-scbist. Sud 
^n^atious arc iiuite analogous to thow auiouj; I'ccciit ii«diiuciitarj' irutteriaU from |>an 
sand, tlirougli muddy saud, aud sandy luud, into niiid or rlay, and betn-ecn sandttaiM 
and Htialra. Thv lliglilands nf Scotland, for iii«taii<.-e, cnibracu large traots of tjuarti- 
HciiUU— rocks tthicli an not |)rojicrly eitlii-r uiica-scliixt or ordiuary iiuartzite. ThFj 
uoiuist of Rnuiular (grant) liti2ed)ijiiartx, with line jionillel lamiine ormiua, aiid are caiabU 
of bctnj; split into thick or thin flogstoLes. InlcrHtratJIiiHl gieblily vari^tJeH ocour. 

Itacoliimitc— a Hchistuse iiuai'tzite, in whidi the quartz -grannies are separated t^ 
line scales of inica, talc, chlorite, mid xi'iieite. Oi'i'iuiouatly these ]i1iable scalca an id 
arranged as to give n certain llexibiiity to the Btoiie (flexilile mndslone). This rod 
occni^ in tliu south ■eastern states of Korth America ; also in Itradl, as the matrix in 
it'hicli dianiondu arc found. 

Siliueuna schist (Lydiun stone, Lydiic, Kio-cl^ehiefer) lias already been deitcribed 
(]). 154] among the sti^tified I'uvks ; but it also occurs among the crystalline scliiib, 
suiiietinies as tliu result of the iinlverisatiun of qliartzosc ■'ockx (niylonite). 

Qnartilte ((jiim-ti-ruck), thongh not jirojicrly n schistose rock, may be most con- 
veniently cunsidcred here, as it is no constant an nccom|>amnieutof the scbuta, and, like 
them, can often be directly traced to tlie alteration of former u.-dinientary fomiatjou. 
It is a granular to coin[Kict mass of c|iiartx, generally wbiti', sometimes yellow or nd 





r t^Adin 



with a cliarocteristic Instivns fructnrc. It occurs in tbiii an<i thick UhIs in 
with i<ehiHts, sometimes in continnuns iiias^s several Ihnusaud feet thick. In Scotland 
it forms rangra of mountains, an<l is there freijui-ntly aL-conijuinied by buds of lime- 
Bloiie, which iu Kntherlandbliirc eontidii Cambrian fosbils.' 

Even to the naked eye, the finely granuUr or arennc'eous structure of qnartrito is 
distinctly visible. Hicniseo])ic exaoiinatiou shows this stnieturc still more clearly, and 
leaves no doubt that the rock originally eoiiiustrd ofa tolerably pure qnartz-sand (Fig. SB]. 
More or lesi distinct evidence of crushing and deformation of the grains may oiten be 
oinierTed. likewise jirouf of the transfusion ofasiliecousci'ineiitnniunK the [articles. Thii 
cement was pmbably ]>iiidnit.'<l by the solvent octiiin of heated water n|>on tlic igiiarti 
grains, which seem to shiule olT into I'ai^h other, or into the iuten-ening silica. It ia 



■ 8(H) tlie chapters on 1 
Uiuiorphic iiuartzose roeki 



PART II § vii SCHISTOSE CRYSTALLINE ROCKS 181 



owing, no doubt, to the purely Riliceous character of tlie grains that the blending of 
these with the surrounding cement is so intimate as often to give the rock an almost 
flinty homogeneous texture. That quartzite, as here described, is an original sediment- 
ary rock, and not a chemical deposit, is shown not only by its granular texture, but by 
the exact resemblance of all its leading features to ordinary sandstone — false-bedding, 
alternation of coarser and finer layers, worm-burrows, and fucoid-casts. Tlie lustrous 
fracture that distinguishes this rock from sandstone, is due to the exceedingly firm 
cohesion of the component gi'ains, which break across rather than separate, and to the 
consequent production of innumerable minute clear vitreous surfaces of quartz. A sand- 
stone, on the other hand, has its grains so loosely coherent that when the rock is broken, 
the fracture passes between them, and the new surface obtained presents innumerable 
doll rounded grains. 

Besides occurring in alternation ninth schists, quartzite is also met with locally as an 
altered form of sandstone, which when traversed by igneous dykes, is indurated for a 
distance of a few inches or feet from the intrusive mass. These local productions 
of quartzite show the characteristic lustrous fracture, and have not yet been distinguished 
by the microscope from the quartz-rock of wide metamorphic regions. There is yet 
another condition under which this rock, or one of analogous structure, may be seen. 
Highly silicated bands, having a lustrous aspect, fine grain, and great hardness, occur 
among the unaltered shales and other strata of the Carboniferous system. In such cases 
the supposition of any general metamorphism being inadmissible, we may infer either 
that these quartzose bands have been indurated, for example, by the passage through 
them of thermal silicated water, or that they are an original formation. 

Bchlitoia CkMDglomerate Bockt. — In some regions of schists, not only bands of 
quartzite occur, representing former sandstones, but also pebbly or conglomeratic bands, 
in which pebbles of quartz and other materials from less than an inch to more than a 
foot in diameter are imbedded in a foliated matrix, which may be phyllite, mica-schist, 
gneiM, quartzite, &c.^ Examples of this kind are found in the pass of the Teto Noire 
between Martigny and Chamouni, in the Saxon granulite region, in the Bergen region of 
Xorway, in the north-west of France, in north -west Ireland, in the islands of Islay and 
Garrelloch, and in Perthshire and other parts of the central Highlands of Scotland. The 
pebbles are not to be distinguished from the water-worn blocks of ordinary conglomerates ; 
bat the original matrix which encloses them has been so altered as to acquire a micaceous 
foliated structure, and to wrap the pebbles round as with a kind of glaze. These facts, like 
those already referred to in the structure of quartzite and argillaceous and quartz-schist, 
are of considerable value in regard to the theory of the origin of some crystalline schists. 

3. Pyroxene-Rocks. — Augite-schist — a fine grained schistose aggregate of pale or 
dark -green augite, with sometimes quartz, plagioclase, magnetite, or chlorite ; found rarely 
among the crystalline schists. Among the schistose rocks of the Taunus, Lossen has 
described some interesting varieties under the name of Augite-schist (Augitschiefer). 
They are green, compact, sometimes soft and yielding to the finger-nail, usually distinctly 
tchistoee, and interbedded with the gneisses and schists. They are composed of a fine 
doll diabase-like ground-mass, through whicli are dispersed crystals of augite, 1 to 2 mm. 
in length, which in the typical varieties are the only components distinctly recognisable 
by the naked eyeu' Augite-rook — a granidar aggregate of augite (with tourmaline, sphene, 
seapolite, etc.), found in beds in the Laurentian limestone of Canada. Malacolite- 
rock is a pale granular to compact, or even fibrous aggregate of malacolite found in beds 

^ Prof. Wichmann describes some curious examples of serpentine conglomerates. See 
his paper in "Beitrage zur Geologie Ost-Asiens und Australieus," ii. pp. 35, 111. On 
the conglomerate -schists of Saxony, see A. Sauer, 'Geol. Si)t;cialkarte Sachsen,' Sect 
" Elterlein," also Lehmann's 'Altkryst. Schiefergesteine,' p. 124. Reusch, ' Silurfossiler 
og Pressede Konglomerater,' Christiania, 1882. Barrois, Anv, Soc. OM, Xord. xi. 1884. 

* Lossen, Zeitich, Deutsch, Geoi, Oes. xix. (1867), p. 598. 



182 ir EOGNt )S Y book ii 



in crystalline limestone (Riescngeliirge). Schistose Gabbro — a granular to seliistoae 
ag^egat* of plagioclase aufl diallage, occurs in lenticular liands among the ainphibolitv 
and gi-anulites of the crystalline schists. The diallage may occur in consjucuous cryAtali, 
and is sometimes associated with ahundant olivine, as in ordinary gabbro (p. 154).* 

These j)yroxenic intei-calations among the schists, like the honiblendic and olivine 
I>ands mentioned below, seem to represent bands of igneous material (lavas or tuft) 
either erupted contemi)oraneously with the dcj^osition of the original material of the 
schists, or 8ubse<iuently intruded into it, and thei-eafter exposed to the metamorphifflD 
which j)roduced the foliation of the si^hists. 

k HuKNDLENDK-RorK!*. — Amphibolltes— a name applied to a grouj> of rocks, con- 
jK)sed mainly of hornblende, sometimes schistose, sometimes thick-bedded. Besides the 
hornblende, numerous other minerals, such as are common among the schists, likewiie 
occur, — orthoclase, j)lagioclase, quartz, augito and varieties, garnet, zoisite, mica, intile, 
&c. Where the rock is schistose, it l)ecomes an amphibolite-schist or homblande-acfaist; 
or if the hornblende takes the form of actinolite, Actinolite-schist. Glaucophane- 
schist — a bluish -grt»y or black rock, in which the hornblende occurs in the form of 
glaucophane, forms large masses in the Southern Al^^, and occiu^ locally in Anglesey. 
Where an am[)hil)olite Ls not schistose, it used to l)e termed hiirnhlcnde-rocJc, Nephrite 
(Jade) is a comi»act, extremely finely fibrous variety. The ju-esence of other minerals b 
noticeable quantity may furnish names for other varieties. Thus, where plagioclsae 
(and some orthoclase) occurs, the rock Iwcomcs a Felspar-amphibolite, Dioritie 
am]>hibolite, or Diorite-schist.'-' Amphiboliti's occur as Itands associated vith 
gneiss and other schistose fonnations. It was suggested by Jukes that they msj 
[)Ossibly rej)resent former l.>eds of hornbh»ndic or augitic lava and tutf, which have been 
metamorphosed together with the strata among which they were intercalated. Tim 
suggestion has received confirmation from the researches of the Geological Survey in 
the north of Scotland and in Ireland, where what were doubtless originally pyroxenie 
masses erupted prior to the metamorphism of the region, have had their augite changed by 
])aramor]>hism into hornblende, and have ]iartially iissumed a foliated structure, iiasstng 
into Epidiorite, Epidiorite-schist, amphibolite-schists, and even serpentine. The 
connection of some schists with original masses of diorite, gabbro, and diabase has been 
]M)inted out by Ijchmann and subseijuently by many other observers.' 

5. (Jaijnkt-Roc'Ks. — Eclogite, one of the most beautiful memlwrs of the crystalline- 
schist series, is a granular aggregate of grass-green omphacite (jiyroxene) and red garnet, 
through which are freipiently disj>ers(Hl bluish kyanit* and white mica. It occurs in 
bands in the Archwan guijiss and mica-schist. To those varieties where the kyanite 
becomes predominant, the name of K yanite-rock has been given. Garnet-rock lit 
crystal line -granular rock composed mainly of garnet, with hoi*nblendc and magnetite; 
by the diminution of the garnet it i»asses into an amphibolite. K inzigite — a crystalline 
schistose rock, com^wsed of jilagioclase, garnet, an<l black mica, found in the Black Forest 
(Kinsig) and the Odenwald. 

^ Kocks of this char.icter occur iu the Saxon **Granulitgt;birge" and also in Lower 
Austria. F. Becke, Tschennak's Mitt. Mifih. IV. p. 352. J. I^ehniauu^s ' Untersuchungen 
iiber die Kntstehuug dor Altkrystallinischen Schiefergesteine,' Bonn, 1884, p. 190. On the 
diabase-schists of the Taunus, see L. Milch, Zeitxch. Dmtsch. Otol. Ges. xli. (1889), p. 394. 

- See F. Becke. Tscherniak's Min. Mitth, IV. p. 233. Tliis author likewise distin- 
guishes diallmjc-a utphilml it€y 'jntiiKt-amphiUdite, saUtc-amphifMiUte^ z(/isiti'-nwphibolUe, 

^ * UntersHchungen iiber die Entstehung der Altkrystall. Schief.' See also Glimhel, 
'Die Paliiolitischeu Eruptivgesteine des Fichtelgebirges, ' Munich, 1874, p. 9 ; Teall, QMorf- 
Juuni. arol. .^>r. xli. (1883), p. 133; 'British Petrography,' p. 198. Hatch, Mem, GtU. 
Surifi/y KxjAnnation of ShfftSf 138, 139, Ireland j p. 49. Hylaud, Mfm, Oeol. Swrveif, 
Kiplanations of North-m'st Ihniq/ol^ oiul nf Smith-vrat /}onff/alf Petrographical appendices* 
also jMmtea, Book IV. pt. viii. G. H. Williams, Jhdi. r..S. f^'eol. Sun: No. 62, 1890. 



PART II § vii SCHISTOSE CRYSTALLINE HOCKS 183 



6. Epidote-Rocks. — Epidofite (Pistacite-rock) — an aggregate of bright green epidote 
with some quartz, occurs with chlorite-schist (Canada), with granite and serpentine 
(Elba), and with syenite. Epidote-schist, a schistose greenish rock, with silvery 
lustre on the foliation surfaces, com]K)sed of epidote, sericite, magnetite, quartz, calcite, 
pla^oclase, and specular iron.^ 

7. CHLORiTE-Rt)CKs. — Chloxite-scliist — a scaly schistose aggregate of greenish chlorite, 
usoally with quartz and often with felsjMir, talc, mica, or magnetite, the last-named 
mineral frequently ap])earing in l)eautifully perfect disseminated octohedra. Occurs \nth 
gneiss and other schists in evenly bedded masses. 

8. Taw-Rocks. — Talc-schiit — a schistose aggregate of scaly talc, often with quartz, 
fels]Mir, and other minerals ; having an unctuous feel, and white or greenish colour. 
Occurs somewhat rarely in beds associated with mica-schist and clay-slate, and frequently 
contains magnetite, chlorite, mica, kyanite, and other minerals, including carbonates. 
A massive variety, com])Osed of a finely felted aggregate of scales of talc, with chlorite 
and serpentine, is called Potstone (Topfstein). Many rocks with a soapy or unctuous 
feel have been classed as talc-schist, which contain no talc, but a variety of mica (sericite- 
schiBt, kc. ) Talc-schist, though not specially abundant, occurs in considerable mass in 
the Aljn (Mont Blanc, Monte Rosa, Carinthia, etc.), and is found also among the A])en- 
nine and Ural mountains. 

9. Olivine -Rot'KS, or Periik>tites pf the Crystalline Schists.^ Rocks of which 
olivine forms a main constituent, occur as subonlinate bands or irregular masses asso- 
ciated with gneisses and other schistose rocks. They were probably eruptive masses, 
contemporaneous with or subsequent to the surrounding gneisses and schists (p. 182 ). 
The olivine is commonly associated with some pyroxenic mineral, hornblende, garnet, kc. 
Some of the rocks mentioned on ]>. 1 73 may also l)e included here. Dunite, for example, 
which occurs in apparently eruptive form at Dun Mountain, near Nelson, New Zealand, 
ia found in North Carolina in beds with laminated structure intercalated in hornblende- 
gneisa. Many of these rocks have undergone much crushing and deformation, and pass 
into foliated forms of Serpentine, which must thus be reckoned as one of the schistose 
as well as one of the eniptive series. Some remarkable schistose serfjentines occur inter- 
bedded among phyllites, mica-schists, and limestones in Baiitrshire. ' 

10. Fklsitoid-Rck'Kh. — These are distinguished by an exceedingly comi>act felsite- 
like matrix. They occur in beds or l)ed-like masses, sometimes in districts of contact 
metamorphism, sometimes associated with vast masses of schists. 

Httltoflinta — an exceedingly comi>act, homstone • like, felsitic, grey, yellowish, 
greenish, reddish, brownish, or black, rock, comi)Osed of an intimate mixture of micro- 
scopic particles of fels[)ar and quartz, ^ith fine scales of mica and chlorite. It breaks 
with a splintery or conchoidal fracture, presents under the miero8coi»e a finely-crystalline 
structure, occasionally with nests of <[uartz, and is only fusible in fine splinters l)efore 
the blow-pipe. Some of the rocks to which this name has been a]»plied are probably 
felaitic lavas ; others, though externally presenting a resemblance to felsite, occur in 
beds intimately associated with foliate<l rocks (Norway), and may be metamorphic 
|)rodacts (perhaps altered tine sediments) due to the same series of changes that gave rise 
to the crystalline schists among which they lie.^ 

Adinola (Adinole-schist) — a rock externally resembling the last, but distinguished 
from it by its greater fusibility. It is an intimate mixture of quartz and albite, con- 
taining about ten ])er cent of soda. It is a ]»roduct of alteration, being found among the 

* See Wichmann on Rocks of Timor, " Beitrage zur Geologie Ost-Asieus uiid Australiens," 
II. part 2, p. 97, Leyden, 1884. 

* See Tschermak, ASUxb. AkcuL Wisscn.^ Vienna, Ivi. (1867). F. Becke, Tschemiak's 
J/i'w. MUth, IV. (1882), p. 322. K. Dathe, Xaies Jahrh. 1876, pp. 255-337. 

* For analyses see H. Santesson, ''Keniiska Bergsartaualyser," 8vo, Stockholn), 
1877. 



184 aj-yxiXuSY BOOKH 



altei-ed Carboniferous shnlcs around the emotive dia^iases of tlie Harz, in tlie altemi 
Devonian rocks of the Taunus, and in the altered Cambrian rocks of South Wales.* 

Porphyroid — a name besto\ve<l u]Km certain rocks comptwied of a felsite-like gronnd- 
mass which has assumed a more or less schistose structure from tlie development of 
micaceous scales, and which contains ]H)r]ihyritically scattered or^'stals of fe1s|iar and 
([uartz. The felsfiar is either orthoclasc or albite, and may Ite obtained in tolertUy 
IKTfect crystals. The quartz occasionally j)i*esents doubly torminatod pyramids. Tbt 
micac'eous mineral may be ]»araj]fonite or sencite. Porphyroid occurs in oircumstaiioei 
which suggest considerable mechanical deformation, as among the schistose rocks of 
Saxony,'-* in the Palfeozoic area of the Ardennes,^ as well as in Westphalia and other 
l«irts of Europe.* Some porphyrf»i<is ai-c probably sheared fonns of quartz-jiorphyiy, 
felsitc, or some similar rock ; othei-s mav be more of the natui-o of tuffs. 

11. <H^ARTZ- AM) T()ri:MALTNK-K(K'Ks. — Tourmaline-BchlBt (Schorl-scliist, schori- 
rock), a blackish, finely granular, (]uart/ose rock i^-ith abundant granules and needles 
of black tourmaline (st-horl), which occurs as one of the products of contact-metamor* 
phism in the neighbourhood of some granites (Cornwall). 

12. QuAKTZ- AM) Mh;a-R<k'Ks. — Mica- schist (Mica -slate, Glimmerschiefer), a 
schistose aggregjite f>f fpiartz and mica, the relative projwrtions of the two minertb 
varjing ^Wdely <;ven in the same mass of rock. Ejich is an-anged in lenticular waTy 
lamiuiv. Tlie (piartz shows gi'cat inconstancy in the number and thickness of its folis. 
It often presents a granular charact<»r, like that of (juartz-rock, or ]>assing into grannlitcL 
The mica lies in thin ]tlates, sometimes so dovetailed into each other as to form long 
continuous irn^gular crumpled folia, separating the quaii;/ layers, and often in the fom 
of thin spangles an<l membranes running in the (piartz. (Figs 8t> and 37.) As the rock 
splits oi>on along its micaceous folia, the i^uartz is not i-eadily seen save in a cross fracture. 

The mica in typical mica-schist is generally a white variety ; but it is sometime* 
replaced by a dark s])ecies. In many lustrous, unctuous schists which are now found 
to have a wide extent, the silvery foliated mineral is ascertained to Ix* a mica (niargaio- 
dite, damounte, etc. ), and not talc, as was onct? supiw^sed. These were named by Dana 
hydro -mica -schists. Among the accessor}' minerals, garnet (specially characteristic), 
schorl, felsiMir, hornblende, kyanite. staurolite. chlorite, and talc may be mentioned. 
Mica-schist n>adily ]»asses into other membei-s of the si;histose family. By addition of 
felsi)ar, it merges into gneiss. Ry loss of quartz and increase of chlorite, it jtasses into 
chlorite-schist, and by loss of mi(ra, into rjuartz-schist and (piart^-ite. By failure of 
([uartz and diminution of micA, with an increasing admixture of calcite, it may shade 
into calc- mica -schist (see Iwlow), and even into marble. Mica -.schist varies in coloor 
mainly accordin*' to the hue of its mira. 

^Ir. Sorby has stated that thin slices of some mica-schists, when examined under 
the niicrosco])e, show traces of original gi-ains of (piartz-siind and other sedimeutaiy 
particdes of which the rock at lirst consisted. He luus also found indications of what he 
su]»i>oses to have been current-bedding «»r ripple-drift, like that seen in many fine sedi- 
mentary deposits, and h<> (concludes that mica-schist is a cr}*stalline metamori^hooed 
sedimentary rock."* In many, if not in mo>«t cases, however, the foliation does not 



^ Lossen, Zvitsrh. Detitsrh. (iod. Uescl. xix. (1867), p. r>73. See also Quart, Journ, iJed. 
•Sfx\ xxxix. (1883), pp. 3i)2, 320. Ro.seubusch, 'Mikroskopische Physiographie/ ii. p. 235. 
F. Posepny, Tschermak's MineruL Mitth, x. 17;"). 

- Rothj^letz, <ii'nl. Surraj Snjonify Explanation of Section Rochlitz. 

■"* De la Valh'*e Poussin and Renard, Mem. (.'ouronnrr'A Armf. Itoy, Belg. 1876, p. 85. 

* Lossen, Sitz. (Mst^Ifsrh. yntvrf. Fnmnie, 1883, No. 9. 

•■* Q. J. (rVrt/. Nw:. (1S63), p. 401, and his address in vol. xxxvi. (1880), p. 85. The 
api)arent cmrent -bedding of many granulitic and other nietaniorphic rocks is certainly 
deceptive, and must be due to planes of shearing or slip]>ing in the mechanical movementi 
which produced the nietauiorphisui. 



PART II § vii SCHISTOSE CRYSTALLINE HOCKS 185 



corresiiond with original bedding, but with stnictural planes (cleavage, faulting) 
sapennduced by pressure, tension, or otherwise, upon rocks which may not always have 
been of sedimentary origin. 

Among the varieties of mica-schist may be mentioned Sericite- schist (which may 
be also included among the phyllites), composed of an aggregate of line folia of the silky 
variety of mica called sericite, in a compact honestone-like ([uartz ; Paragonite-schist, 
where the mica is the hydrous soda variety, paragonite ; Gneiss-mica-schist, con- 
taining dispersed kernels of orthoclase. Some of these rocks contain little or no quartz, 
the place of which is taken by felspar. Calc-mica-schist, a schistose calcareous rock, 
which in many, if not in all cases, was originally a limestone with moi-e or less muddy 
impurity. The carbonate of lime has assumed a granular-crystalline form, while the 
aluminous silicates have re-crystallized as fine scales of white mica. Tremolite, zoisite, 
and other minerals are not infrequent in this rock. 

Normal mica-schist, together with other schistose rocks, forms extensive regions in Nor- 
way, Scotland, the Alps, and other ])arts of £uro]ic, and vast tracts of the ''Archtean" 
regions of North America. Some of its varieties are also found encircling granite 
masses (Scotland, Ireland, etc.) as a zone or aureole of eontact-metamorphism from a 
few yards to a mile or so broad, which shades away into unaltered greywacke or slate 
oatside. In these cases, mica-schist is unquestionably a metamorphosed condition of 
ordinary sedimentary strata, the change being connected with the extravasation of 
granite.' (Book IV. Part VIII.) 

Though the possession of a fissile structure, showing abundant divisional surfaces 
covered with glistening mica, is characteristic of mica-schist, we must distinguish 
between this structure and that of many micaceous sandstones which can be split into 
thin seams, each splendent with the sheen of its mica-Hakes. A little examination will 
show that in the latter case the mica has not crystallized in situ, but exists merely in 
the form of detached worn scales, which, though lying on the same general plane, 
are not welded into each other as in a schist ; also that the quartz does not exist in 
folia but in rounded separate grains. 

13. Quartz- and Felspar-Rooks. — Tlie replacement of the mica of a mica-schist 
by felspar, or the disap[>earance of the mica from a gneiss, gives rise to an aggregate of 
felspar and quartz. Such a rock may be observed in thin liands or courses, alter- 
nating with the surrounding mass. In mineral comi)Ositi()U, it may be com])ared to the 
quartz-porphyries or granite-porphyries of the massive rocks, but it is usually distinguish- 
able by a more or less foliated stnicture, and by the absence of felsitic ground-mass. 

14. Quartz-, Felspar-, axd Mica-Rocks. — GneiBs. — This name, formerly restricted 
to a schistose aggregate of orthoclase (sometimes microcline or a plagioclastic fels]»ar, 
either separate or crystallized together), quartz, and mica, is now commonly em])loyed 
in a wider sense to denote the coarser schists which so often i)re»ent granitoid char- 
acters.^ Many gneisses, indeed, differ from granite chiefly in the foliated arrangement 
of the minerals. The quartz sometimes contains abundant liquid inclusions, in which 
liquid carbon -dioxide has been detected. The relative proportions of the minerals, and 
the manner in which they are grouped with each other, present great variations. As a 
rule, the folia are coarser, and the schistose character less ])erfect than in mica-schist. 
Sometimes the quartz lies in tolerably ]>ure bands, a foot or even more in thickness, with 
plates of mica scattered through it. Tliese quartz layers may be replaced by a crystal- 
line mixture of quartz and felsi)ar, or the felspar will take the form of independent 
lenticular folia, while the laminae of mica which lie so abundantly in the rock, give it 
its fissile structure. The felspar of many gnei.s.ses presents under the microscoije a 



* See Kalkow8ky*s ' Gneissformation des Eulengebirges,' Leipzig, 1878; Lehniaun's 
* Altkrystallinische Schiefergesteine,' 1884 ; F. Becke, Tschermak's Min. Mitth. 1882, i>. 
194 : E. Weber, op. cit. 1884, p. 1, and potttea Book IV. Part VIII. § ii. and Book VI. 
Pre-Cambrian. 



186 GEOGXOSy^ BOOKn 



reinarkablo fibrous structure, duo to tlic cryst«llization of fine lamellio of some pUgio- 
elasK (albite or oligoclasc) in the main mass of orthoclaije or microcline.^ Among tbe 
accc'ssoiy minerals, garnet, tounnaline or schorl, hornblende, apatite, graphite, pyritci, 
and magnetite may be enumerated. 

Tiiere can be no doubt that many gneL^ses owe their characteristic sebistoee stractnif 
to the cnishing and shearing of some original eru]>tive rock such as granite. lustanw^ 
however, occur where the materials ani segi'cgated in bandswhich so closely resemble th<Ne 
of tnie How-structure or segregation in igneous bosse^s and sheets as to suggest that they 
may i>ossibly have resulted from tlie movement of a still unconsolidated eruptive hum 
(pp. 177, 615). Analogies to such structures may l)e observed among ancient and 
mixiern lavas. 

Many varieties of gneiss occur. Some are distinguished by peculiarities of stractnn 
or c'om])08ition, as Granite-gneiss, where the schistose arrangement is so coarse « 
to be unrccoginsable, save in a large mass of the rock ; Diorite -gneiss, gabbro* 
gneiss, comi>osed of the materials of a diorite or gabbro but with a coarsely sebiston 
stnu^ture; Porphyritic gneiss or Augengneiss, in which large eye-like kernels of 
orthoclase or quartz are disixjrsed through a finer matrix and represent larger ciysttb 
or crystalline aggregates whicli have l>een broken down and dragged along by shearing 
movement* in the rock. Other varieties are named from the occurrence in them of mat 
or more distinguishing minerals, as Plornblende-gneiss (syenitic gneiss), in whieh 
hornblende occurs instead of or in addition to mica; Protogine-gneiss, where tbe 
ordinaiy mica is altered into chlorite or a talc-like substance ; Serieitc-gueiss, a 
schistose aggregate of sericite, albite, quartz, with less frequently white and black mioi 
and a chlori tic mineral ;'-^ Augite-gnciss, containing an augitic mineral (not of the 
iliallage group) and potash -fels]>ar or potash-soda-felsi^ar or scai)olite, with hornblende 
(which has often crystallized j>arallel with the augite), brown mica, more or le<s quartx, 
and also frequently with garnet, calcite, titanite, etc. ;^ Plagioclase-gneiss, with 
plagioclase more abundant than oithoclase, sometimes containing hornblende, sometimet 
augite ; Cordierite-gnciss, with the bluish vitreous mineral conlierite. 

The most typical gneisses occur among the so-called ** Archnean rocks,** of which they 
form tlie leading type, and where they probably re])resent original eniptive rocks. (Sfe 
Book VI. Part I.) They cover considerable areas in Scandinavia, N.-W. Scotland, 
Bohemia, Bavaria, Erzge])irge, Moravia, Ontral Alps, Canada, &c. But rocks to whieh 
the name of gneiss c^innot be refused a]>[>ear also among the products of the metanlo^ 
phism of various stratified formations. Such ai-e the gneisses associated with many other 
eiystalline schists among the altered Cambrian and Silurian rocks of Scotland, Norway, 
and New England, the altered Devonian rocks of the Taunus, and other regions, which 
will l)e descril)ed in Book IV. Part \'IIl. Some of these may also l>e eruptive granites, 
diorites, Ac, which have undergone shearing and have acquired a schistose character. 

15. ^Ktaiitz-, Felspar-, ANDGAUNKT-Rfx-KS. — Ghranolite'* (Eurite-schistoide, Lepty- 
nite of French authors, AVeiss-stein) — a fine-grained granular aggregate of i»ale reddish, 

^ F. Becke (Tschernuik's ^fh^. Mitth. 1882 (iv.) p. 198) described this structure and 
named it micritprrthite. 

- K. A. LoHseu, Zeitifch. JfcHfuch. (<lenl. Ges. xix. (1S67), p. r>65. 

^' The occurrence of nugite as an abundant constituent of some gneisses has been made 
known by niicroscoi)ic research. Rocks of this nature occur in Sweden (A. Stelzner, *V. Jakih, 
1880 (ii.), j>. 103}, anil have been fully described from liower Austria (F. Becke, Tscher- 
niak's Min, Mitth. 1882 (iv.), pp. 219-3C5). They are likewise well developed among thi 
oldest gneisses of the north-west of Sutherland in Scotland. 

"* Michel-Levy has proi>0}*ed to reserve the names "Leptynite'' for achistose and 
••Granulite" for erujttive rocks. BulL Sac. Ot<>l. France, 3rd ser. ii. pp. 177, 189, iiL 
p. 287, iv. p. 730, vii. p. 7*>0 : Lory, oj). cit, viii. ]». 14. Scheerer, yeifcs Jahrb. 1878, 
p. C73. Dathe, X. Ja/irb. 1S76, p. 225 ; Z. Da'tsch. O'eol. iifx. 1877, p. 274. Details re- 



PART II § vii SCHISTOSE CRYSTALLINE ROCKS 187 



yellowish, or white felspar with quartz and small red garnets, occasionally with kyanite, 
biotite, and microscopic rutile and tourmaline. The felspar, which is the predominant 
constituent, presents the peculiar fibrous structure referred to in the foregoing descrip- 
tion of gneiss (microperthite, microcline), and api>ears seldom to be true orthoclase. 
The quartz is conspicuous in thin partings between thicker more fels()athic bands, giving 
a distinctly fissile bedded character to the mass. A dark variety, intei-stratified with 
the normal rock, is distinguished by the presence of microscopic augite or diallage 
(Augitgranulite of Saxony). Granulite occurs in bands among the gneiss and other 
members of the crystalline schist series in Saxony, Bohemia, Lower Austria, the Vosges, 
and Central France. The term * * granulite " is also employed in a structural sense to 
denote a rock which has been crushed down by dynamic metamorphism, and has 
acquired this characteristic fine granular structure. (See pp. 99, 119). 

16. Felspar- and Mica-Rocks. — Rocks composed essentially of a schistose aggre- 
gate of minutely scaly mica with some felspar, quartz, andalusite, or other mineral, 
<>ccar in regions of metamorphism. Cornubianite was a name proposed by Boase 
for a rock composed of a felspar base, with abundant mica.* It is found around the 
granite of Cornwall, of which it is a metamorphic product. By some ^Titers this rock 
has been associated with the gneisses, but it Ls distinguished by the scarcity or absence 
of quartz. 



garding the great development of the granulite of Saxony (Granulitgebirge) will be found in 
the explanatory pamphlets published with the sheets of the Geological Survey of Saxony, 
especially those of sections Rochlitz, Geringswalde, and Waldheim. The history of the 
origin of granulite is discussed by J. Lehmann, '' Uutersuchungen iiber die Entstehung der 
AltkrystalL Schiefergesteine." 

1 * Geologj' of Cornwall ' (1832), pp. 226, 230. 



188 



GEOGNOSY 



BOOK II PART n f TJ 





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BOOK III. 

DYNAMICAL GEOLOGY. 

Dynamical GEOixxiY investigates the processes of change at present 
in progress upon the earth, whereby modifications are made on the 
structure and composition of the crust, on the relations between the 
interior and the surface, as shown by volcanoes, earthquakes, and other 
terrestrial distiu'bances, on the distribdtion of land and sea, on the 
outlines of the land, on the form and depth of the sea-bottom, on marine 
currents, and on climate. Bringing before us, in short, the whole 
range of geological activities, it leads us to precise notions regarding 
their relations to each other, and the results which they achieve. A 
knowledge of this branch of the subject is thus the essential groundwork 
of a true and fruitful acquaintance with the principles of geology. The 
study of the present order of nature j)rovides a key for the interpre- 
tation of the past. 

The operations considered by Dynamical Geology may be regarded 
as a vast cycle of change, into the investigation of which the student 
may break at any point, and round which he may travel, only to find 
himself brought back to his starting-point. It is a matter of com- 
paratively small moment at what part of the cycle the inquiry is begun. 
The changes seen in action will always be found to have resulted from 
some that preceded, and to give place to others that follow them. 

At an early time in the earth's history, anterior to any of the periods 
of which a record remains in the visible rocks, the chief sources of 
geological energy probably lay within the earth itself. The planet still 
retained much of its initial heat, and in all likelihood was the theatre 
of great chemical changes. As it cooled, and as the superficial dis- 
turbances due to internal heat and chemical action became less marked, 
the influence of the sun, which must always have operated, and 
which in early geological times may have been more effective than 
it afterwards became, would then stand out more clearly, giving rise to 
that wide circle of surface changes wherein variations of temperature 
and the circulation of air and water over the surface of the earth come 
into play. 



190 DYXAMICAL GEOLOGY book in pabti 



In the 2^ui*8uit of his inquiries into the past history and into the 
present economy of the earth, the student must needs keep his mind 
ever oi>en to the reception of evidence for kinds, and especially for 
degrees, of action which he had not before encountered. Human experi- 
ence has been too short to allow him to assume that all the cauBM 
and modes of geological change have been definitely ascertaiiied. 
Besides the fact that both terrestrial and solar energy were ODoe 
probably more intense than now, there may remain for future discoyeiy 
evidence of former operations by heat, magnetism, chemical change, or 
other agency, that may explain phenomena with which geology has to 
deal. Of the influences, so many and profound, which the sun exerti 
upon oiu* planet, we can as yet only perceive a little. Nor can we tell 
what other cosmical influences may have lent their aid in the revolutioni 
of geology. 

In the present state of knowledge, all the geological energy upon and 
within the earth must ultimately be traced back to the primeval energy 
of the i>arent nebula, or siin. There is, however, a certain propriety 
and convenience in distinguishing between that part of it which is doe 
to the sui-vival of some of the original energy of the planet, and thit 
part which arises from the present supply of energy received day by daj 
from the sun. In the former case, the geologist has to deal with the 
interior of the earth and its reaction upon the surface; in the latter, 
he is called ui)ou to study the surface of the earth, and to some extent 
its reaction on the interior. This distinction allows of a broad treatment 
of the subject under two divisions : — 

I. Hypogene or Plutonic Action — the changes within the 
earth, caused by original internal heat and by chemical action. 

II. E pi gene or Surface Action — the changes produced on the 
superficial parts of the earth, chiefly by the circulation of air and water 
set in motion by the sun's heat. 



Part I. Hypogene Acmox, 

An luqnin/ into the Geological Changes in Progress beneath the Suf/ace 

of the Earth, 

In the discussion of this bi*anch of the subject, it is useful to carry in 
the mind the conception of a globe still intensely hot within, radiating 
heat into space, and consequently contracting in bulk. Portions ol 
molten rocks from inside are from time to time poured out at the fsar- 
face. Sudden shocks are generated, by which earthquakes are propa- 
gated to and along the surface. Wide geographical areas are upraised or 
depressed. In the midst of these movements, the rocks of the crust are 
fi-actured, squeezed, sheared, crumpled, rendered crystalline, and even 
fused. 



i § 1 VOLCANIC PRODUCTS 191 

Section i. Voleanoes and Volcanie Action.^ 

§ 1. Volcanic Products. 

rhe term volcanic action (volcanism or volcanicity) embraces all the 
lomena connected with the expulsion of heated materials from the 
rior of the earth to the surface. Among these phenomena, some 
ess an evanescent character, while others leave permanent proofs of 
r existence. It is naturally to the latter that the geologist gives chief 
Dtion, for it is by their means that he can trace former phases of vol- 
c activity in regions where, for many ages, there have been no vol- 
c eruptions. In the operations of existing volcanoes, he can observe 
" superficial manifestations of volcanic action. But examining the 
» of the earth's crust, he discovers that amid the many terrestrial 
ilutions which geology reveals, the very roots of former volcanoes 
B been laid bare, displaying subterranean phases of volcanism which 
,d not be studied in any modern volcano. Hence an acquaintance 
r with active volcanoes will not afford a complete knowledge of volcanic 
an. It must be supplemented and enlarged by an investigation of the 
es of ancient volcanoes preserved in the crust of the earth. (Book 
Part VII.) 

The word " volcano " is applied to a conical hill or mountain (com- 
sd mainly or wholly of erupted materials), from the summit and often 
i from the sides of which, hot vapours issue, and ashes and streams 
aolten rock are intermittently expelled. The term " volcanic " desig- 
ns all the phenomena essentially connected with one of these channels 
communication between the surface and the heated interior of the 
je. Yet there is good reason to believe that the active volcanoes of 
present day do not afford by any means a complete type of volcanic 

The student is referred to the following general works on the phenomena of volcanoes. 
pe, * Considerations on Volcanoes,' London, 1825; * Volcanoes,' London, 2nd edit. 
>: 'Extinct Volcanoes of Central France,' London, 1858; *0n Volcanic Cones and 
en,' Quart. Juurn. Geol. S(K. 1859. Daubeny, ' A Description of Active and Extinct 
ranoeR,' 2nd edit., Loudon, 1858. Darwin, 'Geological Observations on Volcanic 
idi,* 2nd edit., London, 1876. A. von Humboldt, ' Ueber den Bau und die Wirkung 
Vulkane,' Berlin, 1824. L. von Buch, * Uelx;r die Natur der vulkanischen Erscheiu- 
•n auf den Cauarischen Inseln,' Poggeml. Annalen (1827), ix. x. ; * Ueber Erhebuugs- 
«re nnd Vulkane,' Poggerul. Annalen (1836), xxxvii. K A. von Hoff, *Geschichte 
duicb Ueberlieferuug nachgewiesenen uatlirlichen Verauderungen der Erdobertliiche ' 
tlL, "Vulkane und Erdbeben "), Gotha, 1824. C. W. C. Fuchs, *Dio vulkanischen 
;beiiiiiiigen der Erde,' Leipzig, 1865. R. Mallet, "On Volcanic Energy," PhU. Trans. 
J. J. Schmidt, * Vulkanstudieu,' Leipzig, 1874. Sartorius von Waltershauseu and 
ron Lasaulx, *Der Aetna,' 4to, Leipzig, 1880. E. Reyer, * Beitrag zur Pljysik der 
ptionen,' Vienna, 1877 ; *DieEuganeen ; Bau und Geschichte eines Vulkanes,' Vienna. 
f, Fouque, 'Santorin et ses eruptions,' Paris, 1879. Judd, 'Volcanoes,' 1881. (i. 
r^Wij *Vulcani e Fenomeni vulcanici in Italia,' Milan, 1883. Ch. Velain, ' Les 
:anSy' Paris, 1884. J. D. Dana, 'Characteristics of Volcanoes,' 1890. 'Volcanoes 
; and Present,* E. Hull, 1892. 'The South Italian Volcanoes,' H. J. Johnston-Ljivis, 
les 1891. References will be found in succeeding pages to other and more s|>ecial 

lOilY. 



1 92 D YXA MICAL GEOLOG Y book m pak i 



action. The first effort in the formation of a new volcano is to estaUkk 
a fissure in the earth's crust. A volcano is only one vent or group of 
vents established along the line of such a fissure. But in many parts of 
the earth, alike in the Old World and the New, there have been periodi 
in the earth's history when the crust was rent into innumerable fisflum 
over areas thousands of square miles in extent, and when the moltoi 
rock, instead of issuing, as it does at a modern volcano, in narrow strcami 
from a central elevated cone, welled out from numerous small vents along 
the rents, and flooded enormous tracts of country without forming any 
mountain or conspicuous volcanic cone in the usual sense of these temoA. 
Of these " fissure-eruptions," apart from central volcanic cones, no examples 
appear to have occurred ^nthin the times of human history, except in 
Iceland where vast lava-floods issue<l from a fissure in 1783 (pp. 222, 
256). They can best be studied from the remains of former convulsions. 
Their importance, however, has not yet been generally recognised in 
Europe, though acknowledged in America, where they have been lai^gelj 
developed. Much still remains to be done before their mechanism is as 
well understood as that of the lesser type to which all present volcanic 
action belongs. In the succeeding narrative an account is first presented 
of the ordinary and familiar volcano and its products ; and in § 3, iL, some 
details are given of the general asj)ect and character of fissure-eruptionsL 

The oj^enings by which heiited materials from the interior now 
reach the surface include volcanoes (with their various associated 
orifices) and hot-springs. 

The prevailing conical form of a volcano is that which the ejected 
materials naturally assume round the vent of eruption. The summit d 
the cone is truncated (Figs. 39, 45), and presents a cup-shaped or caldron- 
like cavity, termed the crater, at the bottom of which is the top of tlie 
main funnel or pipe of communication with the heated interior. A 
volcano, when of small size, may consist merely of one cone ; when of 
the largest dimensions, it forms a huge mountain, with many subsidiary 
cones and many lateral fissures or pipes, from which the heated volcanic 
])roducts are given out. Mount Etna (Fig. 39), rising from the sea to* 
height of 10,840 feet, and supporting, as it does, some 200 minor conei^ 
many of which are in themselves considerable hills, is a magnificent 
cxani])le of a colossal volcano.^ 

The materials erupted from volcanic vents may be classed as (1) 
gases and vapoiu*s, (2) wat^jr, (3) lava, (4) fragmentary substances. A 
brief summary under each of these heads may be given here ; the share 
tuken by the several products in the phenomena of an active volcano ii 
<lescribed in § 2. 

^ The structure aud hi:!itory of Etiia are fully described in the great work of SartOTioi 
von Waltcrshauseu and A. von Lasaulx cited on p. 191 — a treasure-honse of ikcti ia 
volcanic geology. See also G. F. llodwell, *Etna, a history of the monntain and tti 
eniption^,' London, 1878; O. Silvcatri, ' Un Viaggio all' Etna,' 1879. Notices of xteanl 
erui>tiou8 of the mountain will be found in J\«^?//y, vols, xix., xx., xxL, xxiL, XIT. 
(o)»servatory on Etna, p. 394), xxvii., xlvi. ; Oimjjf. rend. Ixvi. The work of Mensalli, dltd 
on p. 191, gives descriptions of this aud the other Italian volcanic centres. 



VOLCAKIC GAHBH AND VAFOVKH 



193 



1. Gases and Vapours exist dissolved in the molten magma within 
the earth's crust. They play ua important part in volcanic activity, 
showing themselves in the earliest stages of a volcano's history, and 
continuing to appear for centuries after all other subt«ri-anean action 




has ceased. By much the most abundant of them all is water-^'as, which, 
ultimately escaping as steam, has been estimated to foini y^jVa^hs of 
the whole cloud that hangs over an active volcano (Fig. 40). In great 
eruptions, steam rises in prodigious quantities, and is rapidly condensed 



194 DVXAMICAL GEOLOGY book hi parti 

into a heavy rainfall. M. Fouqui^ calculated that, during 100 days, one of 
the parasitic cones on Etna had ejected vajmnr enough to form, if condenaed, 
2,100,000 cubic mfetrcs (462,000,000 gallone) of water. But even from 
volcanoes which, like the Solfatara of Naples, have been dormant for 
centuries, steam sometimea still rises without intennission and in con- 
siderable volume. Jets of vaponr nish out from clefte in the sides and 
bottom of a crater with a noise like that made by the stoam blown off 
by a locomotive. The number of these funnels or '" fumaroles " is often so 
largo, and the amount of vapour so abundant, that only now and then, 
when the wind blows the dense cloud aside, can a momentary glimpte 
be had of u part of the bottom of the crater ; while at the same time tlic 




rush and roar of the escaping steam remind one of the din of some mt 
factory. Aqueous vajjour rises likewise from rents on the outside of 
the volcanic cone. It issues so copiously from some flowing lavas Uut 
the stream of rock may be almost concealed from view by the cloud ; and 
it continues to escape from Usaurca of the lava, far below the point of 
exit, for a long time after the ixick has solidified and come to rest. So 
satuiHted are numy molten lavas with water-va|)Our that Mr. Scrc^ 
thought that they owed their mobility to this cause,' In the deep vol- 
canic magma the water-substance must be far above its critical temperatan^ 
which is about 773° Kahr. 

Probably in no case is the st«am mere pure vapour of water, thou^ 

when it condenses into copious rain, it is fi-csh and not salt water. It ii 

associated with other vapours and gases disengaged from the potent 

chemical laboratory nndei'neath. There seems to be always a definite 

I ' Cousidiinitious on Volcanoet' (182G), p. IIO. 



5CT. i § 1 VOLCANIC GASES AND VAPOURS 195 

tier in the appearance of these vapours, though it may vary for 
ifferent volcanoes. The hottest and most active " fumaroles," or 
ipour- vents, may contain all the gases and vapours of a volcano, 
It as the heat diminishes, the series of gaseous emanations is reduced. 
bus in the Vesuvian eruption of 1855-56, the lava, as it cooled and 
urdened, gave out successively vapours of hydrochloric acid, chlorides, 
id sulphurous acid ; then steam ; and, finally, carbon -dioxide and 
imbustible gases.^ More recent observations tend to corroborate the 
ductions of C. Sainte-Claire Deville that the nature of the vapours 
reived depends on the temperature or degree of activity of the volcanic 
ifice, chlorine (and fluorine) emanations indicating the most energetic 
liase of eruptivity, sulphurous gases a diminishing condition, and 
xbonic acid (with hydrocarbons) the dying out (of the activity.^ A 
solfatara," or vent emitting only gaseous discharges, is believed to 
IS6 through these successive stages. Wolf observed that on Cotopaxi 
hile hydrochloric acid, and even free chlorine escaped from the summit 

the cone, sulphuretted hydrogen and sulphurous acid issued from the 
iddle and lower slopes. ^ Fouqu^^s studies at Santorin have shown also 
lat from submarine vents a similar order of appearance obtains among 
le volcanic vapours, hydrochloric and sulphurous acids being only found 

points of emission having a temperature above 100° C, while carbon- 
oxide, sulphuretted hydrogen, and nitrogen occur at all the fumaroles, 
'en where the temperature is not higher than that of the atmosphere.*^ 

Tlie following are the chief gases and acids evolved at volcanic fumaroles. Hydro- 
iloric acid is abundant at Vesuvius, and probably at many other vents whence it 
s not been recorded. It is recognisable by its pungent, suffocating fumes, which make 
proach difficult to the clefts from which it issues. Sulphuretted hydrogen and 
Iphurous acid are distinguishable by their odours. The liability of the fonner 
s to decomposition leads to the deposition of a yellow crust of sulphur ; occasionally, 

* C. Sainte-Claire Deville and Leblanc, Ann. Vhim. et Phys.y 1858, Hi. p. 19 et scq. 
T accounts of Vesuvius and its eruptions, besides the general works already cite<l 

p. 191, consult J. PhiUips* * Vesuvius,' 1869 ; 'Mount Vesuvius,' J. L. Lobley, 1889 ; 

Schmidt, *Die Eruption des Vesuv. 1855,' Vienna, 1856; Mercalli's * Vulcani, &c.* ; 
. J. Johnston-Lavis, Q. J, Oed, Soc, xl. 35 ; Geol. Mwj, 1888, p. 445. A diary of the 
Icano's behaviour for six months is given in Naturej xxvi. ; one for four years (1882-1886) 

Dr. Johnston- La vis *Spettatore del Vesuvio,' Naples, 1887 ; a valuable series of reports 

the mountain by the same author will be found in recent volumes of the Reports 
the British Association (1885-91) and a large detailed map of the volcano, also by him, 
blished by Philip, London, 1891. 

^ He distinguished volcanic emanations according to their order of appearance as 
pkrdi time, nearness to the vent, and temperature : viz., 1. Dry fumaroles (without 
tarn), where anhydrous chlorides are almost the only discharge, and where the temjyera- 
re ill very high (above that of melted zinc). 2. Acid fumaroles, with sulphurous and 
drochloric acids and steam. 3. Alkaline (ammoniacal) fumaroles ; temperature about 
0* C. ; abundant steam with chloride of ammonium. 4. Cold fumaroles ; temperature 
low 100** C, with nearly pure steam, accompanied by a little carbon-dioxide, and sometimes 
Iphwetted hydrogen. 5. Mofettes ; emanations of carbon -dioxide with nitrogen and 
ygen, marking the last phase of volcanic activity. 

» SeutsJahrh. 1878, p. 164. * 'Santorin et ses eruptions,' Paris, 1879. 



196 DYNAMICAL (iEOLOCrY book ill pabt i 



also, the production of siilplniric acid is observed Ht active vents. From oWm- 
tions made at Vesuvius in May 1878, Mr. Siemens concluded that vast quantitim of 
free hydrogen or of comhustihle compounds of this gas exist dL»Hi)lve(1 in thf 
magma of the earth's interior, and that these, rising and exploding in the funnels of 
volcanoes, give rise to the detonations and clouds of steam.* At the eniptiun of 
Santoriu in 1S66, the same gases were also distinctly recognisetl by Fouque, who forthr 
fii-sttime established the existence of true volwuiii; Hames. These wei-o again studifd 
s]>ectros(roi)ically in the folli>wing y(?ar by Janssen. who found them to ariMe essentullj 
from the combustion of free hydrogen, but with tmces of chlorine, soda, and copjiw. 
Fou<iuc determined by analysis that, immediately over the focus of eruption, tnt 
hydrogen formed thirty i»er eent of the gases emitted, but that the projM>rtion of tbb 
gas m[)idly diminisluHl with distance from the active vents and Iiotter lavas, while at 
the same time the [iro[>ortion of marsh-gas and carbon -dioxide mpidly increased. Tlif 
gaseous emanations collected by him were foiuul to contain abundant free oxygeu a» 
well as hydrogen. One analysis gjive the following results : carlxin -dioxide 0*22, 
oxygen 21*11. nitrogen "Jl'S^O, hydrogen 56"70, marsh-gas 0*07, = 100*00. Tliift gawoiu 
ifiixture, on coming in contact with a burning bcM.ly, at once ignites with a sharp 
ex])losion. Fouquc infci*s that the water-vajMHir of v<»lcanic vents may exist in a fttste 
of dissociation within th«? molten magma whence lavas rise.- Carbon -dioxide risn 
chieily (a) after an eruiition has ceiised and the volcan<> relajwea into quiescence : or {h) 
after v<»lcanic action has otherwise Income extinct. Of the former ])hase, instances ire 
on reconl at Vesuvius where an eniption has been followed by the emission of this gw 
so copiously from the ground as to suffocate hundreds of hares, pheasants, and i«irtridgn. 
Of the second plui84\ good examples are supplied by the ancient volcanic regions of the 
Eifel and Auvergnc, where the gas still rises in prodigious quantities. Bischof estimated 
that the volume of carbonic acid evolved in the Brohl Thai amounts to 5,000,000 cubie 
feet, or 300 tons of gas in one <iay. Nitrog(;n, derived perhajw* from the decompoa- 
tion of atmospheric air dis.solvcd in the water which iHMietrates into the volcanie foil, 
has been frequently detecteil among the gaseous emanations. At Santorin it was foimd 
to fonn from \ to ^8 ]»cr cent of the gas obtained from ditferent fumaroles.' Fluorinr 
and iodine have likewise been noticed. 

AVith these ga^ses and vaiM)ui-s are associated many substances which, sublimed b)' 
the volwinic heat or resulting from relictions among the escajung vaix>ui'8, apjiear ii 
Sublimates along crevices wherein they reach the air and are cooled. Uesidei 
sulphur, there are several chlorides (imrticularly that <>f sodium, and less alnindaDtlr 
those of potassium, iron, copiwM", and lead); als(> free sulphuric acid, sal- 
ammoniac, specular iron, oxide of copper, boracic acid, alam, sulphate 
of lime, felspars, pyroxene, and other substances. Carbonate of soda occnn in 
large quantities among the fumaroles of Etna. Sodium -chloride sometimes ap]iean » 
abundantly that wide sjuices of a volcanic cone, as well as of the newly-erupted lava, 
ai*e crusted with s;ilt, which can even be ]»r<»titably remove<l by the inhabitants of tBf 
district. Ci»nsiderable i|uantities of chlorMes, &c., may thus Ik; buried between mc- 
cessive sheets of lava, and in long subse^iucnt times, may give rise to mineral spriogi^ 
as has been suggested with reference to the saline waters which issue from volcanic rocks 
of Old Kttd Sandstone and Carboniferous age in Scotland."* The iron-chloride forms i 
bright y(?lli»w and reddish crust on the crater walls, jls well as on loose stones on the 
slojies of the cone. S]>eeular inm, from the decomposition of ii'on -chloride, fonn» 
abundantly as thin lamelhi* in the tissures of Vesuvian lav;us. In the spring of 1873 
the author observed delic^ite l)rown filaments of tenorite (coiqier-oxide, CuO) forming in 
clefts of the crater of Vesuvius. They were u])held by the ui^ti'eaming current of 

^ Mount sb. K. J^rt'uss. A knit. 1878. p. 588. 
■-' Fouque, ' Sautoriu et ses eruptions.' p. 2*25. •' Fouque, loe. ei(. 

* PriH'. Rou. Sihi. Edxn. ix. p. 367. 



SECT, i § 1 WATER IN VOLCANIC ACTION 197 

vapour until blown off by the wind. Fouque has described tubular vents in the lavas 
of Sautorin with crystals of anorthite, sphene, and pyroxene, formed by sublimation. 
Ill the lava stalactites of Hawaii needle-like fibres of breislakite abound. 

2. Water. — Abundant discharges of water accompany some volcanic 
explosions. Three sources of this water may be assigned: — (1) from 
the melting of snow by a rapid accession of temperature previous to or 
during an eruption ; this takes place from time to time on Etna, in 
Iceland, and among the snowy ranges of the Andes, where the cone of 
Cotopaxi is said to have been entirely divested of its snow in a single 
night by the heating of the mountain ; (2) from the condensation of the 
vast clouds of steam which are discharged diuing an eruption ; this 
undoubtedly is the chief source of the destructive torrents so frequently 
obeerv'ed to form part of the phenomena of a great volcanic explosion ; 
and (3) from the disruption of reservoirs of water filling subterranean 
caAities, or of lakes occupying crater-basins ; this has several times 
been observed among the South American volcanoes, where immense 
quantities of dead fish, which inhabited the water, have been swept 
down with the escaping torrents. The volcano of Agua in Guatemala, 
received its name from the disruption of a crater-lake at its summit 
by an earthquake in 1540, whereby a vast and destructive debacle of 
water was discharged down the slopes of the mountain.^ In the 
beginning of the year 1817, an eruption took place at the large crater 
of Idjt'n, one of the volcanoes of Java, whereby a steaming lake of hot 
acid water was discharged Avith frightful destruction doA\Ti the slopes of 
the mountain. After the explosion, the basin filled again ^vith water, 
but its temperature wjis no longer high.- 

In many cases, the water rapidly collects volcanic dust as it rushes 
down, and soon becomes a pasty mud ; or it issues at first in this 
condition from the volcanic reservoirs after violent detonations. 
Hence arise what are termed mud-lavas, or aqueous lavas, which in 
many respects behave like true lavas. This volcanic mud eventually 
consolidates into one of the numerous forms of tuff, a rock which, as 
has been already stated (p. 1.'^^)), varies gieatly in the amount of its 
coherence, in its com})osition, and in its internal arrangement. 
Obviously, unless where subsequently altered, it cannot possess a 
crystalline structure like that of true lava. As a rule, it betrays its 
aqueous origin by more or less distinct evidence of stratification, by 
the multifarious pebbles, stones, blocks of rock, tree-trunks, branches, 
shells, bones, skeletons, &c., which it has swept along in its course and 
preserved within its mass. Sections of this comjxicted tuff may be seen 
at Herculaneum.^ The trass of the Brohl Thai and other valleys in the 

* For an .account of this mountain see K. v. Seebacli, Ahh. fit'si-U. UVs.s-. ar,tfimji',}^ 
xxxviii. (1892) p. 216. 

* See Junghuhu's * Java.' For an account of the volcanoes of the Suiula Islaiul and 
Moluccas, see F. Scheider, Jahrh. Oatf. Rckhsanttt. Vienna, xxxv. (1885). p. 1. Consult 
alno for the Javanese volcanoes the works on Krakatoa (pioU'd on ]». 212. 

' Mallet thought that the so-called ** mud-lavas " of Herculaneum and Ponijieii were not 
aqueous deposits {Journ, Jtot/. ffeoL Sim: Jn'/amf, IV. (1876), p. 144). But there seems no 
reason to doubt that while an enormous amount of ashes fell during the eruption of a.d. 79, 



198 DYNAMICAL GEOLOGY book iii parti 



Eifel district, referred to on p. 137, is another example of an ancient 
volcanic mud. 

3. Lava. — The t^rm lava is applied generally to all the molten 
rocks of volcanoes.^ The use of the word in this broad sense is of 
great convenience in geological descriptions, by directing attention 
to the leiiding character of the rocks as molten products of volcanic 
action, and obviating the confusion and errors which are apt to arise 
from an ill-define<l or incorrect lithological terminology. Precise 
definitions of the rocks, such as those above given in Book II., can 
be added when required. A few remarks regarding some of the 
general lithological characters of lavas may be of service here ; the 
behaviour of the rocks in their emission from volcanic orifices will be 
described in 5^ 2. 

While still flowing or not yet cooled, lavas diflVr from each otiier in the extent to 
which they an^ impregnated with gases and vapour?*. Some appear to be saturated, 
others contain a mnch smaller gaseous im[>regnation : and hence arise important 
distinctions in their behaviour (pp. 217-231). After solidification, lavas present iome 
noticeable charactei-s, then easily ascertainable. (1) Their average specific graritr 
may ^»e taken as ranging between 2*37 and 3-22. (2) The heavier varieties vontUD 
mueh magnetic or titaniferous iron, with augite and olivine, their composition being 
l»asie, and their pro]M)rtion of siliea averaging alx)Ut 4r> to 55 i>er cent. In this groDp 
come the basalts, nepheline-lavas. and leucite-lavas. The lighter varieties contain com- 
monly a minor proiM)rtion of nietallie bashes, but are rich in silica, their ]>ercentage of 
that acid ranging between 70 an«l 75. They arc thus not basic but acid rocks. Among 
their more ini]><)rtant varieties are the rhyolites ami obsidians. Some intermediate 
varieties (trachytes, jihonolites, and andesites) connect the acid and Itasie series. (3) 
Lavas differ mueh in structure and texture, (a) Some are entirely crystalline, consisting 
of an interlaced mass of crystals anrl crystalline p^ntieles, as in some dolerites, and 
granitoid rhyolites. Even quartz, which used to be eonsideretl a non-volcanic minnal, 
cliaracteristic of the older and ehielly of the plutonie eruptive rocks, has ?>een oUserred 
in large crystals in modern lava (1i]Mirite and ijuartz-andcsite '-'). {h) Some show more or 
less of a half-glassy or stony (devitritled) matrix, in which the constituent crystals u« 
imbedded ; this Ls the most common arrangement, (c) Others arc entirely vitreous, sndi 
crystals or crystalline j»articles as occur in them being (piitc subordinate, and, so to 
s[ieak, accidental enclosures in the main glassy mass. Obsidian or volcanic glass is the 
tyiKi of this group, {d) They further differ in the extent to which minute pom or 
larger cellular spaces have been develo[)ed in them. According to Bischof, the {>oroatj 
of lavas dei>ends on their degree of liquidity, a pnous lava or slag, when reduced in lii« 
fusion-exi>erinionts to a thin Mowing eonsistcncy. hardening into a mass as compact u 
the densest lava or basit It.'* Tlie ]>resenee of interstitial steam in lavas, by exjiandiiig 



there were likewise, especially in the later phjises of eruption, copious torrents* of water that 
nilngled with the tine ash and beoame '* mud-lavas." The shar}^)uess of outline and thr 
absence of any trace of alMlondnal ilistension in the moulds of the human bodies found at 
Pompeii, probably show that these victims of the catastrophe were rapidly enveloped in t 
firm coherent matrix which could hanlly have heen mere loose dust. See H. J. Johnston- 
fiavis, ^. ./. Ofol, Svc. xl. p. .Sl>. 

^ '' Alles ist Ijava was ini Vulkaue tliesst und durch seine Fliissigkeit neue Lagerstltter 
einniiumt " is Leopold von Buch's coniprcbcusive definition. 

- Wolf, yevesJohHj. 1874, p. 377. 

^ ' Chem. und Fhys. Geol.' su])ii. (1871), p. 144. On the production of the vesicular 



SECT, i § 1 FRAGMENTARY MATERIALS FROM VOLCANOES 199 

the still molten stone, produces an open cellular texture, somewhat like that of sponge 
or of bread. Such a vesicular arrangement very commonly appears on the upper surface 
of a lava current, which assumes a slaggy or cindery aspect. In some forms of pumice 
the proportion of air cavities is 8 or 9 times that of the enclosing glass. (4) Lavas vary 
greatly in colour and general external aspect. The heavy basic kinds are usually dark 
grey, or almost black, though, on exposure to the weather, they acquire a brown tint 
from the oxidation and hydration of their iron. Their surface is commonly rough and 
ragged, until it has been sufficiently decomposed by the atmosphere to crumble into soil 
which, under favourable circumstances, supports a luxuriant vegetation. The less dense 
lavas, such as phonolites and trachytes, are frequently paler in colour, sometimes 
yellow or buff, and decom^KMse into light soils ; but the obsidians present rugged black 
sheets of rock, roughened with ridges and heaps of grey froth -like pumice. Some of the 
most brilliant surfaces of colour in any rock -scenery on the globe are to be found among 
volcanic rocks. The walls of active craters glow with endless hues of red and yellow. 
The Qrand Ca&on of the Yellowstone River has been dug out of the most mar\'ellously 
tinted lavas and tuffs. 

4. Fragmentary Materials. — Under this title may be included all 
the substances which, driven up into the air by volcanic explosions, fall in 
solid form to the ground — the dust, ashes, sand, cinders, and blocks 
of every kind which are projected from a volcanic orifice. These 
materials differ in composition, texture, and appearance, even during 
a single eruption, and still more in successive explosions of the same 
volcano. For the sake of convenience, separate names are applied to 
some of the more distinct varieties, of which the following may be 
enumerated. 

(1) Ashes and sand. — In many eruptions, vast (quantities of an exceedingly fine 
light grey powder are ejected. As this substance greatly resembles what is left after a 
piece of wood or coal is burnt in an o\f%n lire, it has been (K)pularly termed ash, and this 
name has been adopted by geologists. If, however, by the word ash, the result of com- 
bustion is implied, its employment to denote any product of volcanic action must be 
regretted, as apt to convey a wrong impression. The fine ash-like dust ejected by a 
volcano is merely lava in an extremely fine state of comminution. So minute are the 
particles that they find their way readily through the finest chinks of a closed room, and 
■rttle down upon floor and furniture, as ordinary' dust doe« when a house is shut up. 
From this finest form of material, gradations may be ti*aced, through what is termed 
Tolouiic sand, into the coarser varieties of ejected matter. In composition, the ash and 
sand vary necessarily with the nature of the lava from which they are derived. Their 
microscopic structure, and esjiecially their abundant niicrolites, crystals, and volcanic 
glass, have been already referred to (pp. 136, 137). 

(2) Lapilli or rapilli (p. 136) are ejected fragments ranging from the size of a pea 
to that of a walnut ; round, subangular, or angular in shape, and having the same inde- 
finite range of composition as the finer dust. As a rule, the larger pieces fall neai*est 
the focus of eruption. Sometimes they are solid fragments of lava, but more usually 
they have a cellular texture, while sometimes they arc su light and porous as to float 
rcftdily on water, and, when ejected near the sea, to cover its surface. \Vell-fonncd 
crystals occur in the lapilli of many volcanoes, and are also ejected sei^arately. It has 
been observed indeed that the fragmentary materials not infre<|uently contain finer 
crystals than the accompanying lava.' 

structure consult Dana, 'Characteristics of Volcanoes,' p. 161. Compare also Judd, Oeol. 
Mng, 1888, p. 7. 

' Sartorins von Waltershausen, 'Sicilien mid Island,' ISf'B, p. 328. 



200 



DYXAMIfAL OEOLOUY 



(3) Voluonii: Blocks (]i. 136) arc larger pipces of stone, often aiif^l&r in ihape. Ii 
Koiue vane* they ati[<PAr to lie frnpiiriits loowurd fmiii nlready solidified rocki in tkl 
rliimiwy oftlie voivaiio. Uriicc we liiid atiioug them \aecta of non-volcaiiic nx^ If 
well IB of older tulfs and Iovsh recogiiiiiaMy lii'loiiginfc to early eniptioiut. In many 
vases, tliey are fjevted in enoriiioUB i[uantitieH ilurint; the earlier pliaiwa of rioleit 
eriiiitioii. T]ie great exi>loHio]i from the Hide of Amrot in 18J0 U'ut Bcvompuiied lif 
tlic diKelmr;^ of a vaxt i^uanlity of ftKgineiitH over a tijiaco of many square niilea amold 
the nioiiiitaiii. Whitney han ilewKlied the occurreura in California of beds of od 
fragmentary volcailio lirecfin, linndredx of feet thiek and covering niuiy sqiiaro mileirf 
Hurfiiee. Junghidin, in hiH ai-coiint of the eni^ition in Java in 177S, nieutiona that a 
valley ten inileH long was filled to an nvenige depth of Hfty feet with angular Tokanie 

Among the earlier eruiitioiiH of u volcano, fraginentH of the rocks through nliicb tk( 
vi'Tit has 1>ct'n drilletl may fii-iineiitly he ohiierved. These txv in many caaes not Tolcanie. 
lilockH of ajliiat and granitoid rockn occur in tlic cinder-licdg at the baae of the voleaaie 
Hcries r)f Santoriii. In thr oUlei' tulfK iif Soninia, [lieces of altered liinostouo (Bonutuna 
iiieasnrilig 200 i-illnc feet or more anil weighing upwards of IS tons) orp abuudant and 
often [contain cavities lineil with the eliami'terintic: " Viwuvian mineralH." ' Bloclci of a 
coarsely (TyHtnlliuc granitoid (Init really tmchytic} lava liai'e been [larticularly obKmd 
lioth on Etna' and VennviuB. In the year ISiO a niasx of that kind, weighing Herenl 
tonn, WOK to W seen lying at llie foot of the njijier cone iif Vesuvius, witliiu the entnaee 
to the Atrio ilel Cavallo. Similar lilocki occur among tlie Carboaiferoua volcajiie pipM 
of iTeutral Hcotlaiiil, tugetliersonietinieH with fragments of santbitone, shale, or limeitona, 
not ill frequently full of Carboniferous fowiln.'' Enonuoua nuuses of variouB schists han 
Win tarricl up liy the lavas uf the Tertinry volcanic plateau of the Inner Hebrides ' 

(4) \ ol a Do sad slag — Tl ese ) a onginallj formed portiona of tk 
-ot of la a lUK-e 1 g tl e ] Ijir of a olta o aud 1 a been dctacl ed • d hniU 

t tie ar > H -euH c j In f ntvn \ (Fg -11 is a roolli 




' But nee the reiuarlu alreaily made on volcanic cougloiiiersteH, mtlt, p. ISM. 
- See H. J. JolmHtan-f-ivis y. J. iJoJ. f-e. xl. p. 7H. 

' For till' eruiiteil blocks (AnswilrHinge) of Etna see ' Der Aetua,' IL ppL 211, 
330, 4»1. 

* TfBU*. Iby. Hk. RUh. x\\*. p. -1.19. Seejiiw'rt'. Book IV. Sect. vii. 1 1. i. 
' Trans, llry. .So,-. Rli«. XXXV. (1888,i, [^ 82. 



BBCT. i § 1 FHAGMEXTARY MATERIALS FROM VOLCAXoES 201 



elliptical, or i)ear-.s}ja|»ed, often iliscoidal mass of lava, from a few inclies to s<>veral feet 
ill diameter ; sometimes tolerably solid throughout, more usually coarsely cellular in- 
iiide. Xc)t iiifre<iueutly its interior is hollow, and the bomb then consists of a shell 
which ia most close-grained towards the outside, or the centre is a block of stone 
with an external coating of lava. There can be no doubt that, when torn by eructations 
of steam from the surface of the boiling lava, the material t»f these Iwmbs is in 
mK thoroughly molten a condition as the rest of the mass. From the rotatory 
uiotion im|)arted by its ejection, it takes a circular fonn, an<l in i)ro})ortion to its 
rapidity of rotation and fluidity is the amount of its "flattening at the j»ole.s." The 
centrifugal force within allows the expansion of the interstitial vapour, while the outer 
Hurfiu'e rapidly cools and solidities ; hence the solid crust, and the ^jorous or cavernous 
interior. Such Ijomlw, varying from the size of an apjile to that of a man's body, were 
found by Darwin abundantly strewn over the gi-ouml in the Island of Ascension ; they 
were ako ejected in vast <[uantities during the eruption of Santorin in 1866.^ Among 
the tulfa of the Eifel region, small bombs, consisting mostly of gi-anular olivine, are of 
common occurrence, as also jueces of wuiidine or other less fusible minerals which have 
Mfp^egfttcd out of the magma liefore ejection. In like manner, among the tuffs tilling 
volcanic necks, pi*ol»ably o( Pennian age, which pierce the C'arlM)niferous rocks of Fife, 
large worn cr}'stals of orthoclaae, biotite, Ac, are found. AVhen the ejected fragment 
of lava has a rough irregidar form and a (lorous stnu-tuie. like the clinker of an iron- 
furnace, it is known as a slag." 

.The fragmentary materials crupteil by a volcano and dejK>8ited around it aC4iuire by 
degrees more or less consolidation, j>artly from the mei-e pressure of the higher \i\)on 
the lower strata, jiartly from the influence of infiltrating water. It has l)een already 
stated (p. 136) that different names are applied to the rocks thus formed. The 
coarf«e, tumultuous unstratified accumulation of volcanic jlcbris within a crater or 
funnel is called Agglomerate. When the <lcbris, though still coarse, is more 
roimde«1, and is arranged in a stratified form on the sloiies of the cone or on the country 
Iteyond, it becomes a Volcanic Conglomerate. The finer-giuine<l varieties, formed 
of dust and lapilli, are included in the general designati(Mi of Tuffs. Those are 
iwiially pale yellowish, greyish, or brownish, sometimes black n^ks, giJinular, porous. 
and often incoherent in texture. They occur interstratified with and pass into ordinary' 
non-voloanic Mdiment. 

Orgudc remains 8<imetimes tMvnir in tuff. AVhen* volcanic debris has aicuniulate<l 
OTier the floor of a lake, or of the sea, the entombing and ))res4'rving of .shells an4l other 
oripilic objects must continually take place. Kxani)»les of this kind are lited in later 
|Hgea of this volume from older geological formations. IVofe.ssor (iui.scardi of Na])les 
foand about 100 species of marine shells of living sjkk-Ics in the old tuffs of 
VesDTioa. Marine shells have been picked up within the «iater of Monte Nuovo, and 
have been frequently ol>serve<l in the old or marine tuff of that district. Showers of 
aMh, or Hheets of volcanic mml, often pivserve land-shells, in.sect*, and vegefcition living 
on the area at the time. The ohler tuffs of Vesuvius have yielde<l many remains of the 
shrubs and trees which at sucoes.sive jM*ri(Kls have clothed the flanks of the mountain. 
Fragments of coniferous woo<l, which once grew on the tutl'-«-ones of Carlioniferous 
age in i-entral Scotland, are abundant in the *'ne(*ks" of that regi<»n, while the minute 
stnicture of some of the lepi<lo4lendroid plants has also been admirably ]»n'served there 
in tutf.^ 



* Darwin, 'Geological Obser>'ations on Volcanic Islands." 2n(l e»lit. p. 12. Fou(iu«'', 
* Santorin/ \k 79. 

* On the ratio Wtween the pores and volume nf the rock in slagx and lavas, see 
determinations by Bischof, 'C'heni. und Phys. (Jeol.' sup|i. (lf<71). p. IAS. 

* TrtiH*. Rtiif, iSfJT, Etiin. xxix. p. 470; t^usteo. Hook IV. Tart VII. Sect. ii. § 2. 



202 DYNAMICAL GEOLOGY book m pami 



§ 2. Volcanic Action. 

Volcanic action may be cither constant or peiiodic. Stromboli, in 
the Mediterranean, so far as we know, has been uninterruptedly emitting 
hot stones and steam, from a liasin of molten lava, since the earlittt 
I>eriod of history.^ Among the Moluccas, the volcano Sioa, and in tlie 
Friendly Islands, that of Tofua, have never ceased to be in eruption since 
their first discovery. The lofty cone of Sangay, among the Andes of 
Quito, is alwavs giving off hot vapours ; Cotopaxi, too, is ever constantly 
active.^ But, though examples of unceasing action may thus be cited 
from widely different quarters of the globe, they are nevertheless ex- 
ceptional. The general rule is that a volcano breaks out from time to 
time with varying vigour, and after longer or shorter inter\'als ol 
quiescence. 

Active, Dormant, and Extinct Phases. — It is usual to class volcanoei 
as adive, donnant^ and exiinci. This arrangement, however, often presenti 
considerable difficulty in its application. An active volcano cannot of 
course be mistaken, for even when not in eruption, it shows by its 
discharge of steam and hot vapoure that it might break out into activity 
at any moment. But in many cases, it is im^xissible to decide whetlier 
a volcano should l>e called extinct or only dormant. The volcanoes of 
Silurian age in Wales, of Car]>onifei'ous age in Ireland, of Permian age 
in the Harz, of Miocene age in the Hebrides, of younger Tertiary age in the 
Western States and Territories of North Ameriai, are certainly all extinct 
But the older Tertiary volcanoes of Iceland are still represented there 
by Skai)tAr-r7okulK Hecla, and their neighlwurs.^ Somma, in the fint 
centiu*y of the Christian era, would have been naturally regarded as in 
extinct volcano. Us tires had never been known to have been kindled; 
its vast crater was a wilderness of ^^-ild vines and brushwood, haunted, no 
doubt, by wolf and wild boar. Yet in a few days, during the autumn of 
the year 79, the half of the crater walls was blown out by a terrific series 
of explosions, the present Vesuvius was then formed within the limita of 
the earlier crat^M", and since that time volcanic action has been inter- 
mittently exhibited up to the present day. Some of the intervak of 

* For accounts of StroiiOwili sec Si>alltanzaiii's 'VoyogL's dans les deux Siciles.* ScTOpe's 
' Voloanops. ' Ju«M, Oo>l. Mmj. lS7r». Mercalli's * Vulcani, &o.* p. 135 ; ami bis pspeniB 
Atti i<in:. ItiiL Si'i. Sat. xxii. xxiv. xxvii. xxix. xxxi. L. W. Kulclier, ^»V*i/. J/rrijr. 1890,p.34i* 

- For descriptions of (!otopaxi» hee Wolf, iWitva Jnhrh. 1,S7S ; Whyuji>er, Sai^rt^ nin. 
p. ri'J3 : * Travels amongst tlie Great Andes,' cha]). vi. 

^ On the volcanic plK^uoniena of Iceland consult G. Mackenzie's 'Travels in the blud 
of Iceland during the Summer of 1810.' K. Hemlersou's 'Iceland.' Zirkel, *De gwg* 
nostica Islandu' constituti(me olKservationes,' Bonn. 1S61. Tlioroddsen, 'Ovenigt owde 
islandske Vulkauers Historie,* translat«Ml in resiune by (4. H. Boehmer, SmithsoHtaH M. 
Jirp. 1885, part i. p. 495 ; also ni/tnn;/ t. Srenak. Vft. Akmi, Hanff/. 14, ii. (1888). 17, ti- 
(1891) : O'eoi. Ma,j. 1S80, p. -IfiS : Suture, Oct. 1884. MUlh. K. K. Urogr, Oe*, Vienna, 
xxiv. (1891), p. 117. Keilhack, /eitsrh, Deutwh. ii»:oK Citwf. xxxviii. (1886). p. 376; 
Schmidt, np. cif. xxxvii. (1885), p. 737; A. Helland, 'Lakis Kratere og Lavm-atriime,' 
rnicrrAifefs /*nM/rinutii''j Christiania, 1885; Bruon. Mrj'ologie do I'lAlande, et des lik« 
Foeroe," Paris, 1884 ; T. Anderson. Joftru. St. Arts, vol. xl. (1892). p. 397. 



SECT, i § 2 VOLCANIC ACTION 203 

quietude, however, have been so considerable that the mountain might 
then again have been claimed as an extinct volcano. Thus, in the 131 
years between 1500 and 1631, so comj^tely had eruptions ceased that 
the crater had once more become choked with copsewood. A few pools 
and springs of very salt and hot water remained as memorials of the 
former condition of the mountain. But this period of (]uiescence closed 
with the eruption of 1631, — the most powerful of all the known ex- 
plosions of Vesuvius, except the great one of 79. In the island of 
lachia, Mont' Epomeo was last in eruption in the year 1302, its previous 
outburst having taken place, it is believed, about seventeen centuiies 
before that date. From the craters of the Eifel, Auvergne, the Vivarais, 
and central Italy, though many of them look as if they had only recently 
been formed, no eruption has been known to come during the times of 
human history or tradition. In the west of North America, from Arizona 
to Oregon, numerous stupendous volcanic cones occur, but even from the 
mo6t perfect and fresh of them nothing but steam and hot vapours has 
yet been known to proceed. ^ But the presence there of hot springs and 
geysers proves the continued existence of one phase of volcanic action. 

In short, no essential distinction can be drawn ]>etween dormant 
and extinct volcanoes. Volcanic action, as will be afterwards pointed 
oat, is apt to show itself again and again, even at vast intervals, within 
the same regions and over the same sites. The dormant or waning 
condition of a volcano, when only steam and various gases and sublimates 
are given off, is sometimes called the Solfatara phase, from the well- 
known dormant crater of that name near Naples. 

Sites of Volcanic Action. — Volcanoes may break through any geo- 
logical formation. In Auvergne, in the Miocene period, they burst 
through the granitic and gneissose plateau of central France. In 
Lower Old Eed Sandstone times, they pierced contorted Silunan rocks 
in central Scotland. In late Tertiary and j)ost-Tertiary ages, they found 
their way through recent soft marine strata, and formed the huge piles 
of Etna, Somma, and Vesuvius ; while in North America, during the 
same cycle of geological time, they flooded with lava and tuff many of 
the river -courses, valleys, and lakes of Nevada, Utah, Wyoming, Idaho, 
and adjacent territories. On the banks of the Rhine, at Bonn and else- 
wherCy they have penetrated some of the older alluvia of that river. In 
many instances, also, newer volcanoes have appeared on the sites of older 
ones. In Scotland, the Carboniferous volciinoes have risen on the ruins 
of those of the Old Red Sandstone, those of the Permian period have 
broken out among the earlier Carboniferous eruptions, while the older 
Tertiaiy dykes have been injected into all these older volcanic masses. 
The newer pays of Auvergne were sometimes erupted through much older 
and already greatly denuded basalt-streams. Somma and Vesuvius have 
risen out of the great Neapolitan plain of older marine tuff, while in 
central Italy, newer cones have been thrown up ui)on the wide Roman 
plain of more ancient volcanic debris.- The vast Snake River lava-fields 

* firnpikms oocnrred perhapsjess than 1 00 years ago. I )iller, linU. U. .s'. (»<•///. .sv rr. . No. 79. 
' Accofding to Professor G. Pozzi, the principal volcanic outbursts of Italy are of 



204 DYXAMICAL HEOLOtrY book m pami 



of Idaho overlie deiiucled masses of earlier trachytic lavas, and similir 
])roofs of a long succession of intermittent and \videly-se]>arated volcanic 
outbursts can be traced northwanls into the Yellowstone Valley. 

When a volcanic vent is ojKjned, it might l>e supposed always to 
find its way to tlie siu'face along some line of fissure, valley, or deep 
dei)ression. No doubt many, if not most, modern as well as anciail 
vents, especially those of large size, have done so. It is a curious fad, 
however, that in innumei*al»le instances minor vents have appeared when 
there was no visible line of dislocation in the rocks at the surface to aid 
tliem. This has been well shown bv a study of the ancient volcanie 
rocks of the Old Red Sandstone, Carboniferous, and Permian formatioH 
of Scotland.^ It has likewise been most impressively demonstrated bf 
the way in which the minor iKisalt cones and craters of Utah hm 
broken out near the edges or even from the face of cliffs, rather thai 
at the bottom. Ca])Uiin Button remarks that among the high plateaux 
of Utah, where there are hundreds of basaltic craters, the least 
l)lace for them is at the base of a cliff, and that, though they occur 
faults, it is almost always on the lifted, rarely upon the depressed side.' 
On a small scale, a similar avoidance of the valley bottom is shown 
on the Khine and Moselle, where eru})tions have taken place cloai 
to the edge of the plateau through which these rivera wind. WTiy out- 
breaks should have occurred in this way is a question not easily answered. 
It suggests that the existing depressions and heights of the earth*! 
surface may sometimes be insignificant features, com|)arcd with the deplk 
of the sources of volcanoes and the force employeil in volcanic emptioo. 
On the other hand, it is remarkable that in Scotland the Palseoioie 
erui)tions took place on the low gi'ound and valleys, and continued to 
show themselves there during a long succession of volcanic periodL 
Ksi)ecially noteworthy is the way in which the Permian vents were opened 
in lines and groui)s along the bottom of long narrow valleys in the 
Silurian uplands.' 

Ordinary phase of an active Volcano. — Tlie interval between two 
eruptions of an active volcano shows a gi*jidual augmentation of eues^. 
The crater, emptied by the last discharge, has its floor slowly upnueed 
l>v the expansive force of the lava-column underneiith. Vajwurs rise in 
constant outflow, accompanied sometimes by discharges of dust or stonefti 
Through rents in the ciat<?r-floor red-hot lava may l>e seen only a few feet 
flown. Where the lava is maintained at or above its fusion-point and 
possesses great lirjuidity, it may form l)oiling lakes,- as in the great crater 
of Kilauea, where acres of seething lava may be watched throwing up 
fountains of molten rock, surging agjiinst the walls and re-fiisiug laige 
masses that fall into th(^ buining flood. The lava-column inside the ppe 

tlic Glacial IVrioil. Atfi Linrei^ .Snl ser. vol. ii. (1878\ p. 35. Stefaiii regards thoM of 
Tuscany as partly Miocene, partly Pliocene an»l post-Plioi'enc. [Pruc. TVwc. <S>r. Sat. Piitt 
1. p. xxi.) 

* T I'll lis. Iimj. Siu\ Etiiii. xxix. i». 437. 

■' " Ilijjli Plateaux of I- tali," Urnl. muJ f^>fff. .Si'nr.t/ nf TfiYtforifJt, 1880, p. 62. 

^ t^iiort, Jiiiirn. (r\'ni. S»r. vol. xlviii. (18P2). l*resi«lential Addresftf p. 156. 



BBCT. i § 2 VOLCANIC ACTION 205 



of a volcano is all this time gi-adiially rising, until some weak jwrt of 
the wall allows it to escape, or until the pressure of the accumulated 
vapours becomes great enough to biust through the hardened crust of the 
crater-floor and give rise to the phenomena of an eruption. 

Conditions of Eruption. — Leaving for the present the general 
question of the cause of volcanic action, it may be here remarked that 
die conditions determining any i)articular eruption are still unknown. 
The explosions of a volcano may l)e to some extent regulated by the 
conditions of atmospheric ])ressiu'e over the area at the time. In the 
eftBO of a volcanic fimnel like Stromboli, where, as Scroi)e pointed out, 
the expansive subterranean force within, and the repressive effect of 
atmospheric pressure without, just balance each other, any serious dis- 
turfoance of that pressure might be expected to make itself evident by 
a change in the condition of the volcano. Accordingly, it has long been 
remarked by the fishermen of the Lifmri Islands that in stormy weather 
there is at Stromboli a more copious discharge of steam and stones than 
in fine weather. They make use of the cone as a weather-glass, the 
increase of its activity indicating a falling, and the diminution a rising 
harometer. In like manner, Etna, according to Sartorius von Walt^rs- 
hansen, is most active in the winter months. Mr. Coan has indicated 
a relation between the eruptions of Kilauea and the rainy seasons of 
Hawaii, most of the discharges of that crater taking place within the four 
months from March to June.^ 

When we remember the connection, now indubitably established, 
between a more copious discharge of fire-damp in mines and a lowering 
of atmospheric pressure, we may be prei>ared to find a similar influence 
affecting the escape of vajwui-s from the upper surface of the lavu-colunui 
of a volcano ; for it is not so much to the lava itself as to the ex|>ansive 
▼apours impregnating it that the miuiifestations of volcanic activity arc 
due. Among the Vesuvian eruptions since the middle of the seventeenth 
century, the number which took place in winter and spring has l)een to 
that of those which broke out in summer and autumn as 7 to 4. In 
Japan, also, the greater numl>er of recorded eruptions have taken place 
during the cold months of the year, February to April. - 

There may be other causes besides atmospheric pressui-e concerned in 

' DaoA, 'Characteristics of Volcanoes/ p. 125. For accounts of the volcanic phenomena 
of Hawaii, see W. Ellis, 'Polynesian Researches.' Wilkes' U.S. Exjiloring Expedition, 
1838-42, •*Geolog}'," by J. D. Dana. The Rev. T. Coan, a missionary resident in Hawaii, 
obeerred the operations of the volcanoes for upwanls of forty years, and publislted from time 
to time short notices of them in the Amerimn Journal n/ Sriettre^ vols. xiii. (1852) xiv. 
XT. xriiL xzi. zxii. xxiiL xxv. xxvii. xxxvii. xl. xliii. xlvii. xlix. ; 3rd ser. ii. (1871) iv. vii. 
▼iiL xiT. xriii. xx. xxi. xxii. (1881). Prof. Dana has recently revisited these volcanoes and 
fully diMossed their phenomena in the Autcr. Jnurti. N-«'. vols, xxxiii.-xxxni. (1887-89 1, 
and in his 'Characteristics of Volcanoes.' See also C. E. Dutton, Amrr. Jonr/i. S-i. 
XXT. (1883), p. 219; Rfjntrt C.K Oeoli^jicnl Surcetj, 1882-83. L. Green, 'Vestiges of 
the Molten Globe/ 1887. For an account of the remarkable glassy lavas of Hawaii, see 
E. Cohen, Sevea Jahrb. 1880 (ii.), p. 23 ; and a general account of the petrograi)]iy of 
the islands, by E. S. Dana, Amer. Join-n. Sci. xxxvii. (1889\ p. 441. 

* J. Milne, Seismot. Sue, Japan, IX. Part ii. p. 174. 



206 



DVXAMICAL GEOLOGY 



BOOK UIPARl 



those dirteronoos ; the preiwnderance of rain during the winter and spnn{ 
may Ih» one (»f these. Accoi-ding to Mr. Coan, pre^ioiis to the grol 
llawaian eruption of 18(58 there had lK»en unusually wet weather, aid 
to this fact he attri1»utes the exceptional severity of the earthquakes ui 
voleanie explosions. The p'eater fre<iueney of Japanese volcanic eruptiooi 
and eai-thipiakes in winter ha.< been referred in explanation to the fact liyt 
the avenii^e Imrometrie gradient across Jaixin is steeper in winter thank 
summer, while the piling up of snoAv in the northern regions gives rise to 
long-iH.»ntinueil stresses, in conse«|uence of which certain lines of weaknoi 
in the earths crust are more prejKired to give way during the vrintff 
months than they are in summer.^ The etlects of vai'jnng atmosphetie 
pressure, however, can pii^lwhly only slightly and locally modify volcuk 
activity. Kni]>tions. like the gre-at one of (.'otopaxi in 1877, have ifl 
inmuneniMe instances taken place without, so far as can be ascertained, 
any ivterence to atmospheric cv^nilitions. 

Klui;e has souirht to trace a connection between the vears of nuudnim 
and mininnim suii-sjk»i^ and those of givatest and feeblest volcanie 
.kctivitv, and ha^^ constnicted lists to show that vears ^which hava 
Ivcn sivcially characteiiscvi by terrestrial eruptions have coincided with 
Those marked by few sun-sj>»ts and diminished magnetic. distiirbanoeL' 
Such a conticction caunor Iv reg-ardetl as having yet been satisfactorilf 
establisheil. Ag-ain, the siune auth«>r hascalletl attention to the frequency 
and \igi»ui' of volcanic explosions at or near the time of the August 
meteorii- sh«.»wci-. l^ut i:i this case, likewise, the cited examples cu 
iianllv vet U' li^^ked u;v!i as more iha!i i oiiu-iilences. 

PeriodieUy of Eruptions. — At mai^.v volcanic vents the eruptiTe 

^■!'.c-.g\' m;r..i!cs:s ::sclt wi:h m«i:v «•'.• loss rei:ularity. At 8trombot^ 

\xhi».h is iov.s:a!'.!'v ir; a!i .u::vi' s:a:o. :hr oxi^i'.»T?it.!is v»ccur at intennb 

^.ivy::'.i: ti>^!u th'.vo •:■ !■•.;:■ :•■ :tv. :i::!v.;:o< ar.d upwanls. A simikr 

• hyrh:v.iv,il v.i'xcmcr^: h,is -t-:: i::cv. '"-irrve^l during the eruptions at 

orhcr \cv> \*:::vh .*rx. :•.•»: o ■!>:a!-.:'.y ac::ve. Volcano, for example^ 

.::•::•.;: ::- cv.i;>:>*:i .'f S^^vrcr.i'**:- l>7:>. .l:s;nayt^i a succession of ex- 

:'■ 's-i^Ts wl-.\h :.>**.o\\i\: t\ijh -rhcv a: :r.:<rv,i> i.f from twentv to thirtr 

\: Kv.i .iT-Ai V^s*.:\"/.:'i .i <i:v.:\ir rhythmical series of convul- 

ry-::-. •'•'i^vvr,: .l::-"!:'^ :he course of an eruptioiL' 

\*.-. ■_ :•'., \ ..■■ v< v^:" :V.c A-.io^ .-. ;.vr' -•■..• ..ii-H-bArge of steam has beoi 

' ■ w -. '. 'J- 1 . M • \\ :• V :v.:\ v ' •. . : i . -. vl * ::: •■ :<h l s • : s:e;iir. :• • proceed at intervak 

:" :■■•■. :*.^. "tv :o -.rvirTy !:::.*::ts ::> v.-. :r:: s::v.r.::: ».i Ninpii, while daring 

•. > i ■ > ' K '. : : : ':•. -. ^ -.wi : -, : vi : ^ r ■ :" \. *•. : ; : v* x : . : h is % •>lcaLno iras seen to 

At the eniptioD 



.... . .« « ^ 



> \o c "*'.::> h.i< 



■w 



. k- 



.1 .o 






■* i * I*' ".iTir 



Mr. Milne observed 



* . . ■ ". ■ . I*" 



>?/- y. ri A. Ptaey {ComfUi 
^1=1131 is M2ar $poCt. Sec, 



j?:L ■.^. Ti. IW. 



SECT, i § 2 VOLCANIC ACTION 207 

that the explosions occurred nearly every two seconds, with occasional 
pauses of 15 or 20 seconds.^ Kilauea, in Hawaii, seems to show 
a regular system of grand eruptive periods. Dana has pointed 
out that outbreaks of lava have taken place from that volcano at 
intervals of from eight to nine years, this being the time required to fill 
the crater up to the point of outbreak, or to a depth of 400 or 500 
feet2 

Some volcanoes have exhibited a remarkable paroxysmal phase of 
activity, when after comparative or complete quiescence a sudden gigantic 
explosion has taken place, followed by renewed and prolonged repose. 
Vesuvius supplies the most familiar illustration of this character of 
volcanic energy. The great eruption of A.D. 79, which truncated the 
upper part of the old cone of Somma, was a true paroxysmal explosion, 
unlike anything that had preceded it within historic times, and far more 
violent than any subsequent manifestation of the same volcano. The 
crater-basin of Santorin, of which the islands Thera and Therasia represent 
portions of the rim, seems to have been blown out by some stupendous 
paroxysm in prehistoric times. The vast explosion of Krakatoa in 1883 
was another memorable example. In these instances there was an earlier 
period of ordinary volcanic activity, during which a large cone was gradu- 
ally built up. In the case of Somma and Krakatoa the energy died down 
for a time, and the paroxysm came with hardly any premonitory warning. 
It has been succeeded by a time of comparatively feeble activity. At 
Vesuvius the great explosion of 1631, which terminated nearly 1500 years 
of quiescence, may be regarded as a minor paroxysm, since which the 
mountain has remained more continuously active. 

General sequence of events in an Eruption. — The approach of 
an eruption is not always indicated by any premonitory symptoms, for 
many tremendous explosions are recorded to have taken place in different 
parts of the world without perceptible warning. Much in this 
respect would appear to depend upon the condition of liquidity of 
the lava, and the amount of resistance offered by it to the passage of 
the escaping vapours through its mass. In Hawaii, where the lavas 
are remarkably liquid, vast outpourings of them have taken place 
quietly without earthquakes during the present century. But even 
there, the great eruption of 1868 was accompanied by violent earth- 
quakes. 

The eruptions of Vesuvius are often preceded by failure or diminu- 
tion of wells and springs. But more frequent indications of an approach- 
ing outburst are conveyed by sympathetic movements of the ground. 
Subterranean rumblings and groanings are heard ; slight tremors succeed, 
increasing in frequency and violence till they become distinct earthquake 
shocks. The vapours from the crater grow more abundant, as the lava- 
column in the pipe or funnel of the volcano ascends, forced upward and 
kept in perpetual agitation by the passage of elastic vapours through its 

* Traru. Seitm. Soc. Japan y ix. part ii. p. 82. 

* 'Characteristics of Volcanoes.' p. 124. On periodicity of eruptions, see Kluge, Neuea 
Jahrb, 1862, p. 582. 



208 DYXAMICAL (GEOLOGY book in part i 



mass. After a long previous interval of quiescence, there may l>e much 
solidified lava towards the toj) of the funnel, which will restrain the 
ascent of the still molten portion underneiith. A vast pressiu'e is thiiB 
exercised on the sides of the cone, which, if too weak to resist, will 
open in one or more rents, and the liquid lava will issue from the outer 
slope of the mountain ; or the energies of the volcano will be directed 
towards clearing the obstruction in the chief throat, until with tremend- 
ous explosions, and the rise of a vast cloud of dust and fragments, the 
bottom and sides of the crater are finally blown out, and the top of the 
cone disa])i)ears. The lava may now esciipe from the lowest part of the 
lip of the crater, while, at the siime time, immense numbers of red-hot 
lx)mbs, scori*, and stones are shot up into the air. The lava at 
first rushes down like one or more rivers of melted iron, but, as it 
cools, its rate of motion lessens. Clouds of steam rise from its surface, 
as well as from the central crater. Indeed, every successive paroxysmal 
convulsion of the mountain is marked, even at a distance, by the rise of 
huge ball-like wieaths or clouds of steam, mixed with dust and stones, 
forming a column which towers sometimes a couple of miles or more 
above the summit of the cone. By degi'ees these erucUitions diminish in 
frequency and intensity. The lava ceases to issue, the showers of stones 
and dust decrease, and after a time, which may vary from hours to days 
or months, even in the re(iime of the same mountfiin, the volcano becomes 
once more trantjuil.^ 

In the investigation of the subject, the student Avill naturally devote 
attention specially to those aspects of volcanic action which have 
more particular geological interest from the permanent changes with 
which thev are connected, or from the way in M'hich thev enable 
us to detect and realise conditions of volcanic energy' in former 
periods. 

Fissures. — The convulsions which culminate in the formation of 
a volcano usually split open the terrestrial crust l»y a more or less nearly 
rectilinear hssure, or by a system of fissures. In the subsequent progress 
of the mountain, the ground at and around the focus of action is liable 
to 1)C again and again rent open by other fissures. These tend to diverge 
from the focus ; but around the vent where the rocks have been most 
exposed to concussion, the fissures sometimes intersect each other in aU 
directions. In the great eruption of Etna, in the year 1669, a series of 
six parallel Assurers opened on the side of the mountain. One of these, 
with a width of two yards, i-an for a distance of 12 miles, in a somewhat 
winding course, to within a mile of the top of the cone.'^ Similar fissures, 
but on a smaller scale, have often been observed on Vesuvius and other 
volcanoes.^ A fissure sometimes reoi>ens for a subsecpient eruption. 

^ See Schmidt's narrative of the eruptiou of Vesuvius in May 1855 {anUy p. 195), An 
account of the ijreat eruption of Cotopaxi in June 1877, by Dr. Th. Wolf, will be foand in 
XeuefiJuhrh. 1878, p. UX 

^ For tissures on Etna, sm* Silvestri, lh»n. R. O'enf. Com. Ital. 1874. 

^ For a description of those of Iceland (which run chiefly N.K to S.W., aud N. to S.) 
see T. Kjerulf. A'.y/. Mog. xxi. 147. 



i§2 



vowAsif^ rissuims 



Two obvious causes may be assigned for the pushing upward of a 
crater-floor and the fisauring of a volcanic cone : — (1) the enormous pressure 
of the dissolved vapours or gases acting upon the walls and roof 
of the funnel and convulsing the cone by successive explosions; and (2) 
the hydrostatic pressure of the lava-column in the funnel, which may be 
taken to be about 120 lbs. per square inch, or nearly 8 tons on the square foot, 
for each 100 feet of depth. Both of these causes may act simultaneously, 
and their united effect has been to uplift enormous superincumbent masses 
of solid rock and to produce a wide-spread series of long and continuous 
fissures reaching from unknown depths to various distances from the sur- 
face and even opening up sometimes on the surface. These results of the 
expansive energy of volcanic action are of special interest to the geologist, 
for he encounters evidence of similar operations in former times preserved 
in the crust of the earth (see Book I\'. Part '\'II. Sect i.) 

Into rents thus formed, the water-substance or vapour rises with 
great expansive force, followed by the lava which solidifies there like iron 
in a mould. Where fissures are vertical or highly inclined the igneous rock 
tekes the form of d^ixs or reins ; where the intruded material has forceil 
its way more or less in a horizontal direction between strata of tuff, beds 
of non-volcanic sediments, or flows of lava, it takes the form of uluets {siUs) 
or bedi. The cliffs of many an old crater show how marvellously they 




have been injected by such veins, dykes, or sheets of lava. Those of 
Sonuna, and the A'alle del Bovc on Etna {Fig. -12), which have long been 
known, project now from the softer tuffs like wulla of masonry.' The 

* a. von WRltersliaasen, ' Dcr Aetna.' li. p. 341. 



210 inW'AMICAL GKULOOY hook hi rsxti 

cniter cliffs of Stintorin also present iiii abundant series of dykes. The 
|)cnnaiieiit separation of thu walln of fissures by the consolidation d 
the luv« that rises in them iw dykes must widen the dimensions of i 
lono, for tlie fissures are not Awn to shrinkage, although doubtless the 
looKcly i>ilod fi-ajpiiciitary materials, in the course of their consolidatioa, 
develop lines of joint Sometimes the lain has evidently risen in a stale 
of oxtrcmu fluidity, and has at once tilled the rents prepared for it, cool- 
ing rapidly on the outside as a true volcaniu ginss, but assuming a da- 
tinctly crystalline structure inside (mil'-, p. 171). Dykes of this kind, 
with a vitreous crust on their sides, may bo seen on the crater-vall of 
Sonitna, and not uncommonly among liasalt dykes in Iceland and Scotland 
In other cases, the lava had probably already ac<|uircd a more viscous « 
even litlioid clmructer ere it ruse in the tissure, and in this condition wm 




ibh to pn-'h a"! lo and i 
made Its 1% Li (Fi„ 41) 




Ln lontort the itrata of tnlf through which it 
Theie laii bt littk d inbt that in the archibc- 
mw^t let tlic part of huge beams and gudoi 
(^l), 4 i) liniijnig the loose tufTs and ml* 
c. diite 1 la\Hst (.ethir and sti cngthening the 
tone if^ui-'t the offctts of subsequent coo 
\ ul*ioii« 

From this jioitit of Mon an explanation 
xuf^ctv itidf (if the obsencd altemationi in 
Kt 41 ■*«( of ink™ ifi-i thechiu'-tcter of a\o1ianoBimption8. TbcM 
i""^!,!"'''''"" ' " dtewiation-* nia\ d(|>cnd m groat meaioR 

•upon the iclition between the height of 
the tone, on thi one haiiil and the stiength of ita sides, on the other 
When the sides hiM liecn vtoll hraied togetbLr b\ interlaung d\keB,uid 
fnrthc r thi<.kcn< d b\ the spn-ul of ^ olciiiic matenals all o^ er their slopn 
the^ m i\ resist the eHctts of explosion and of the pressure of the ascend- 
ni{, Kia-column In thH e \t^ the voleiiiin may find relief only from it» 
Kumniit "knd if the 1a\i flont forth, it will do so from the top of the 
tone Vs the (one ineref<c-' in elevation, however, the pressure from 
within uiKin it-< ^de- ancient'. Kventiially egress is once more estab- 
li-,hed on the flanks hi inc-iiH of fissures, and a new series of Uva- 
strenmR is poured out u^ er the lower sloiies (jimfeii, p. 248). 



L ftEXT. i§2 VOLCANIC EXPLOSIONS 211 



■ In the deeper portions of a volcanic vent the convulsive efforts of the 

E lava-column to force its way upward must often produce lateral as well 
I as vertical rifts, and into these the molten material will rush, exerting as 
■f it goes an enormous upward pressure on the mass of rock overlying it. 
At a modern volcano these subterranean manifestations cannot be seen, 
but among the volcanoes of Tertiary and older time they have been 
revealed by the progress of denudation. Some of these older examples 
teach us the prodigious upheaving power of the sheets of molten rock in- 
truded between volcanic or other strata. An account of this structure 
(sills, laccolites), with reference to some examples of it, will be found in 
Book IV. Part VII.^ 

Though lava very commonly issues from the lateral fissures on a 
volcanic cone, it may sometimes approach the surface in them with- 
out actually flowing out. The great fissure on Etna in 1669, for 
example, was visible even from a distance, by the long line of vivid 
light which rose from the incandescent lava >vithin. Again, it 
frequently happens that minor volcanic cones are thrown up on the 
line of a fissure, either from the congelation of the lava round the point 
of emission, or from the accumulation of ejected scoriae round the 
* fissure-vent. One of the most remarkable examples of this kind is that 
of the Laki fissure in Iceland, the whole length of which (12 miles) 
bristles with small cones and craters almost touching each other.^ 

Explosions. — Apart from the appearance of visible fissures, volcanic 
energy may be, as it were, concentrated on a given point, which will 
usually be the weakest in the structure of that part of the terrestrial crust, 
and from which the solid rock, shattered into pieces, is hurled into the 
air by the enormous expansive energy of the volcanic vapours.^ This opera- 
tion has often been observed in volcanoes already formed, and has even been 
witnessed on ground previously unoccupied by a volcanic vent. The history 
of the cone of Vesuvius brings before us a long series of such explosions, be- 
ginning with that of a.d. 79, and coming down to the present day (Fig. 45). 
Even now, in spite of all the lava and ashes poured out during the last 
eighteen centuries, it is easy to see how stupendous must have been that 
earliest explosion, by which the southern half of the ancient crater was blown 
out. At every successive important eruption, a similar but minor operation 
takes place within the present cone. The hardened cake of lava forming 
the floor is burst open, and with it there usually disappears much of the 
apper part of the cone, and sometimes, as in 1872, a large segment of 
the crater-wall. The Valle del Bove on the eastern flank of Etna is a 
chasm probably due mainly to some gigantic prehistoric explosion.* The 
islands of Santorin (Figs. 65 and 66) bring before us evidence of a pre- 
historic catastrophe of a similar natiu*e, by which a lai'ge volcanic cone 

^ See particularly the descriptiou of intnisivo sheets or laccolites. ^ 

* A. Helland, 'Lakis Kratere og Lava-stroiue,' cited on p. Ii02. On this straight 
fiMtire some 500 craters rise, varying from 5 to 450 feet high. 

* See Daabr^'s experiments on the mechanical effects of gas at liigh i»ressures, Comptes* . 
Rend. cxL cxlL cxiii., and Bull, Soc. Ge^d. France^ xix. (1891), p. 313. 

* ' Der Aetna,' p. 400. 



212 nyXAMWAL OEOLOGY book ill part I 

was blown up. The existing outer islands aie a chain of fragments of 
the ]>erii>hery of the cone, the centre of which ia now occupied by the sea. 
In the year 1538 a new vnU-ano, Monte Xuovo, was formed in 24 hours 



on the margin of the Bay of Naples. An opening was drilled by suc- 
cessive explosions, and such (juantitJes of stones, scoriae, and ashes were 
thrown out from it as to form a hill that i-osc -140 English feet above the 
sea-level, and was more than a mile and n half in circumference. Moet 
of the fragments now to be seen on the sloiws of this cone and inside its 
beautifully perfect crater arc of various volcanic rocks, many of them 
being black scoriai ; but pieces of Koman potteiy, together with frag- 
ments of the older underlying tuff, ami some marine shells, have been 
obtained — doubtless part of the soil and subsoil dislocated and ejected 
during the explosions. 

One of the most stupendous volcanic explosions on record was that 
of Krakatoa in the Sumla Strait on the 26th and 27th of August 1883,' 
After a series of eon\Tilsions, the greater portion of the island was blown 
out with a succession of terrific detonations which were heard more than 
1.50 miles away. A mass of matter, estimated at about 1^ cubic mile in 
bulk, was hurled into the air in the foi'm of lapilli, ashes, and the finest 
volcanic dust. The efi"ects of this volcanic outburst were marked both 
upon the atmosphere and the ocean. A series of barometrical disturbances 
[xissed n^innd the glolie in opposite directions from the volcano at the rate 
of about 700 miles an hour. The air-wave, travelling from east to west 
is supposed t<) have passed three and a quarter times round the earth (or 
82,21)0 miles) Iwfoi'e it ceased to be perceptible.- The sea in the neigh- 
bourhood was thrown into waves, one of which was computed to have 
risen more than 100 feet above ti<le-lcvet, destroying towns, villages, and 
36,380 people. Oscillations of the water were perceptible even at Aden, 

' 9ee'Th»Ernptic.nofKratalon,'b)-aComniitte«oflheBoya1Societ)-, 1888. 'Knkatan,' 
11. I>. M. Verbtck, Batavin, 1888. 

' Sratt ami Strachey, /'rvc. Roy. Soc. ixivi. (18831. Eojnl Society's Report, p. 67. 



SECT, i § 2 VOLCANIC DUST AND STONES 213 

1000 miles distant, at Port Elizabeth in South Africa, 5450 miles, and 
among the islands of the Pacific Ocean, and they are computed to have 
travelled with a maximum velocity of 467 statute miles in the hour.^ 

It is not necessary, and it does not always happen, that any actual 
solid or liquid volcanic rock is erupted by explosions that shatter the 
rocks through which the funnel passes. Thus, among the cones of the 
extinct volcanic tract of the Eifel, some occur which consist entirely, or 
nearly so, of comminuted debris of the surrounding Devonian greywacke 
and slate through which the various volcanic vents have been opened 
(see pp. 200, 244, 585). Evidently, in such cases, only elastic vapours 
forced their way to the surface ; and we see what probably often takes 
place in the early stages of a volcano's history, though the fragments of 
the underlying disrupted rocks are in most instances buried and lost 
under the far more abundant subsequent volcanic materials. Sections of 
small ancient volcanic "necks" or pipes sometimes afford an excellent 
opportimity of observing that these orifices were originally opened by 
the blowing out of the solid crust and not by the formation of fissures. 
Examples will be cited in later pages from Scottish volcanic areas of Old 
Red Sandstone, Carboniferous, and Permian age. The orifices are there 
filled with fragmentary materials, wherein portions of the surrounding and 
underlying rocks form a noticeable proportion. ^ (See pp. 584-589). 

Showers of Dust and Stones. — A communication having been opened, 
either by fissuring or explosion, between the heated interior and the surface, 
fragmentary materials are commonly ejected from it, consisting at first 
mainly of the rocks through which the orifice has been opened, afterwards 
of volcanic substances. In a great eruption, vast numbers of red-hot 
stones are shot up into the air, and fall back partly into the crater and 
partly on the outer slopes of the cone. According to Sir W. Hamilton, 
cinders were thrown by Vesuvius, during the eruption of 1779, to a 
height of 10,000 feet. Instances are known where large stones, ejected 
obliquely, have described huge parabolic curves in the air, and fallen at 
a great distance. Stones 8 lbs. in weight occur among the ashes which 
buried Pompeii. The volcano of Antuco in Chili is said to send stones 
flying to a distance of 36 (?) miles, Cotopaxi is reported to have hurled 
a 200-ton block 9 miles,^ and the Japanese volcano, Asama, is said to 
have ejected many blocks of stone, measuring from 40 to more than 100 
feet in diameter.* 

But in many great eruptions, besides a constant shower of stones and 
scoriae, a vast column of exceedingly fine dust rises out of the crater, 
sometimes to a height of several miles, and then spreads outwards like 
a sheet of cloud. The remarkable fineness of this dust may be understood 
from the fact that during great volcanic explosions no boxes, watches, or 

* Wharton, Royal Society's Report, p. 89. For a great Japanese explosion, see Nature, 
13th Sept 1888. 

* Trans. Roy. Soc. Edin. xxix. p. 458 ; Quart. Jnurn. Geol. Soc. (1892), President's 
Address, pp. 86, 118, 135, 143, 153. 

' D. Forbes, Oeol. Mag. vii. p. 320. 

* J. Milne, Seism. Soc. Japan y ix. p. 179, where an excellent account of the volcanoes of 
Japan is given. See also ' The Volcanoes of Japan * by J. Milne and W. K. Burton. 



214 D\ '^A MICA L GEOLOGY book iii part i 



/ 



close-fitting joints have been found to be able to exclude it. Mr. 
Whymper collected some dust that fell 65 miles away from Cotopaxi, 
and which was so fine that from 4000 to 25,000 particles were required 
to weigh a grain.^ So dense is the dust-cloud as to obsciu^e the sun, and 
for days together the darkness of night may reign for miles around the 
volcano. In 1822, at Vesuvius, the ashes not only fell thickly on the 
villages round the base of the mountain, but travelled as far as Ascoli, 
which is 56 Italian miles distant from the volcano on one side, and as 
Casano, 105 miles on the other. The eruption of Cotopaxi, on 26th 
June 1877, began by an explosion that sent up a column of fine ashes 
to a prodigious height into the air, where it rapidly spread out and formed 
BO dense a canopy as to throw the region below it into total darkness.^ So 
quickly did it diffuse itself, that in an hour and a half, a previously bright 
morning became at Quito, 33 miles distant, a dim twilight, which in the 
afternoon passed into such darkness that the hand placed before the eye 
could not be seen. At Guayaquil, on the coast, 150 miles distant, the 
shower of ashes continued till the 1st of Jul v. Dr. Wolf collected the 
ashes daily, and estimated that at that place there fell 315 kilogrammes 
on every square kilometre during the first thirty hours, and on the 30th 
of June, 209 kilogrammes in twelve hours.*'^ During a much less im- 
portant eruption of the same mountain on 3rd July 1880, the amount of 
volcanic dust ejected, according to Mr. Whymper, could not have been 
less, and was probably vastly more, than two millions of tons,* equivar 
lent to a mass of lava containing more than 150,000 cubic feet. 

The explosion of Kmkatoa in August 1883 was accompanied by 
the discharge of enoraious quantities of volcanic dust, some of which 
was carried to vast distances. It was estimated that the clouds of 
fine dust were hurled from that volcano to a height of 17 miles, and 
the darkness which thev caused extended for 150 miles from the 
focus of eruption. The diffusion and continued suspension of the finer 
particles of this dust in the upper air has been regarded as the prob- 
able cause of the remarkably brilliant sunsets of the following winter 
and spring over a large part of the earth's surface.^ One of the most 
stupendous outpourings of volcanic ashes on record took place, after 
a quiescence of 26 years, from the volcano Coseguina, in Nicaragua, 
during the early jjart of the year 1835. On that occasion, utt^r darkness 
prevailed over a circle of 35 miles radius, the ashes falling so thickly that, 
even 8 leagues from the mountain, they covered the gi^ound to a depth 
of about 10 feet. It was estimated that the rain of dust and sand fell 

* Tloyal Society Report ou Krnkatoa, p. 183. 

- During the comparatively insignificaut eruption of this volcano in 1880 Mr. AVTiymper 
noticed that a column of inky blackness, fonneil doubtless of volcanic dust, went stra^ht 
up into the air with such velocity that in less than a minute it had risen 20,000 feet above 
the rim of the crater, or 40,000 feet alwve the sea. 'Travels amongst the Great Andes,' 
I». 322. 

'* Xeuci Jalii'h. 1878. p. 141. An account of this eruption is given by Mr. Whymper in 
his ' Travels amongst the Great Andes,* chap. vi. 

"* 'Travels amongst the Great Andes.' p. 328. 

** Royal Society Report, pp. ir»l-463. 



SECT, i § 2 VOLCANIC D UST 2 1 5 



over an area at least 270 geographical miles in diameter. Some of the finer 
materials, thrown so high as to come within the influence of an upper 
air-current, were borne away eastward, and fell, four days afterwards, at 
Kingston, in Jamaica — a distance of 700 miles. During the great eruption \ 
of Sumbawa, in 1815, the dust and stones fell over an area of nearly one 
million square miles, and were estimated by Zollinger to amount to fully 
fifty cubic miles of material, and by Junghuhn to be equal to one hundred 
and eighty-five mountains like Vesuvius. Towards the end of last cen- 
tury, during a time of great disturbance among the Japanese volcanoes, 
one of them, Sakurajima, threw out so much pumiceous material that 
it was possible to walk a distance of 23 miles upon the floating debris in 
the sea. 

An inquiry into the origin of these showers of fragmentary materials 
brings vividly before us some of the essential features of volcanic action. 
We find that bombs, slags, and lapilli may be thrown up in comparatively 
tranquil states of a volcano, but that the showers of fine dust are dis- 
charged with violence, and only appear when the volcano becomes more 
energetic. Thus, at the constantly, but quietly, active volcano of Strom- 
boli, the column of lava in the pipe may be watched rising and falling 
with a slow rhythmical movement. At every rise, the surface of the lava 
swells up into blisters several feet in diameter, which by and by biu'st 
with a sharp explosion that makes the walls of the crater vibrate. A 
cloud of steam rushes out, carrying >vith it hundreds of fragments of the 
glowing lava, sometimes to a height of 1200 feet. It is by the ascent 
of steam through its mass, that a column of lava is kept boiling at the 
bottom of the crater, and by the explosion of successive larger bubbles of 
steam, that the various bombs, slags, and fragments of lava are torn oflT 
and tossed into the air. It has often been noticed at Vesuvius that each 
great concussion is accompanied by a huge ball-like cloud of steam which 
rushes up from the crater. Doubtless it is the sudden escape of that 
steam which causes the explosion. 

The varying degi*ee of liquidity or viscosity of the lava probably 
modifies the force of explosions, owing to the different amounts of 
resistance offered to the upward passage of the absorbed gases and 
vapours. Thus explosions and accompanying scoriae are abundant at 
Vesuvius, where the lavas are comparatively viscid ; they are almost 
unknown at Kilauea, where the lava is remarkably liquid. 

In tranquil conditions of a volcano, the steam, whether collecting into 
larger or smaller vesicles, works its way upward through the substance 
of the molten lava, and as the elasticity of this compressed vapour over- 
comes the pressure of the overlying lava, it escapes at the surface, and 
there the lava is thus kept in ebullition. But this comparatively quiet 
operation, which may be watched within the craters of many active 
volcanoes, does not produce clouds of fine dust. The collision or friction 
of millions of stones ascending and descending in the dark column above 
the crater must doubtless cause much dust and sand. But the explosive 
action of steam is probably also an immediate cause of much trituration. 
The aqueous vapour or water-gas which is so largely dissolved in many 



216 DYXAMICAL GEOLOGY book hi part i 

lavas must exist within the lava-cohimn, luider an enormous pressure, at a 
temperature far above its critical point (p. 194), even at a white heat, and 
therefore possibly in a state of dissociation. The sudden ascent of lava 
so constituted relieves the pressure rapidly >nthout sensibly affecting the 
temi)orature of the mass. Consequently, the white-hot gases or vapours 
at length explode, and reduce the molten mass to the finest powder, like 
water shot out of a gun. ^ 

Evidently no part of the operations of a volcano has greater geological 
significance than the ejection of such enormous quantities of fragmentary 
matter. In the first place, the fall of these loose materials round the 
orifice of discharge is one main cause of the growth of the volcanic cone. 
The heavier fragments gather around the vent, and there too the thickest 
accumulation of dust and sand takes place. Hence, though successive explo- 
sions may blow out the upper part of the crater-walls and prevent the 
mountain from gi'owing so rapidly in height, every eruption must increase 
the diameter of the cone. In the second place, as every shower of dust 
and sand adds to the height of the ground on which it falls, thick volcanic 
accumulations may be formed far beyond the base of the moiuitain. The 
volcano of Sangay, in Ecuador, for instance, has buried the country around 
it to a depth of 4000 feet under its ashes.- In such loose deposits are 
entombed trees and other kinds of vegetation, together with the bodies of 
animals, as well as the works of man. In some crises, where the layer of 
volcanic dust is thin, it may merely add to the height of the soil, without 
sensibly interfering with the vegetation. But it has been observed at 
Santorin that though this is tnie in dry weather, the fall of rain with the 
dust at once acts detrimentally. On the 3rd of June 1866, the vines 
were there withered up, as if they had been burnt, along the track of the 
smoke cloud.^ By the gi-adual accumulation of volcanic ashes, new 
geological formations arise which, in their component materials, not only 
bear >vitness to the volcanic eruptions that produced them, but preserve 
a record of the land-surfaces over which they sjM'ead. In the third place, 
besides the distance to which the fragments may be hurled by volcanic 
explosions, or to which they may be diffused by the ordinary aerial 
movements, we have to take into account the vast spaces across which the 
liner dust is sometimes borne by upixjr air-ciUTents. In the instance 
already cited, ashes from Coseguina fell 700 miles away, having been 
carried all that long distance by a high counter-current of air, moving 
ai)parently at the rate of about seven miles an hour in an opposite direc- 
tion to that of the wind which blew at the surface. By the Sumbawa 
eruption, also referred to above, the sea west of Sumatra was covered with 
a layer of ashes two feet thick. On several occasions ashes from the Ice- 
landic volcanoes have fallen so thickly between the Orkney and Shetland 
Islands, that vessels passing there have had the unwonted deposit 

* Messrs. Murray aud Reiiard (/Voc. Itoy. S^^r. EiUn. xii. (1S84), p. 480) concluded that 
the fragnieutary condition aud the fresh fractures of the dust -particles of the Krakatoa 
eruption were due to a tension plienonienon, which affecU these vitreous matters in a manner 
analogous to what is observed in '* Rupert's drops." 

- D. Forbes, Gfol. Mag, vii. 320. ^ Fouqut*, 'Santorin,' p. 81. 



SECT, i § 2 LA VA'STREAMS 2 1 7 

shovelled off their decks in the morning. In the year 1783, during the 
memorable eruption of Skaptar-Jokull, so vast an amount of fine dust 
was ejected that the atmosphere over Iceland continued loaded with it 
for months afterwards. It fell in such quantities over parts of Caithness 
— a distance of 600 miles — as to destroy the crops ; that year is still 
spoken of by the inhabitants as the year of " the ashie." Traces of the 
same deposit have been observed in Norway, and even as far as Holland.^ 
Hence it is evident that volcanic accumulations may take place in regions 
many hundreds of miles distant from any active volcano. A single thin 
layer of volcanic detritus in a group of sedimentary strata would not 
thus of itself prove the existence of contemporaneous volcanic action in 
its neighbourhood. Failing other proof of adjacent volcanic activity, it 
might have been Avind-borne from a volcano in a distant region. 

Lava^streams. — At its exit from the side of a volcano, lava 
glows with a white heat, and flows with a motion which has been 
compared to that of honey or of melted iron. It soon becomes red, 
and like a coal fallen from a hot fireplace, rapidly grows dull as it moves 
along, until it assumes a black, cindery aspect. At the same time the 
surface congeals, and soon becomes solid enough to support a heavy block 
of stone. The aspect of the stream varies with the composition and 
fluidity of the lava, form of the ground, angle of slope, and rapidity of 
flow. Viscous lavas, like those of Vesuvius, break up along the surface 
into rough brown or black cinder-like slags and irregular ragged cakes, 
bristling with jagged points ("a<t"2j^ which, in their onward motion, 
grind and grate against each other with a harsh metallic sound, sometimes 
rising into rugged mounds or becoming seamed with rents and gashes, at 
the bottom of which the red-hot gloAving lava may be seen (Fig. 46). In 
lavas possessing somewhat greater fluidity, the surface presents froth-like, 
curving lines, as in the scum of a slowly flo^ving river, or is arranged in 
curious ropy folds, as the layers have successively flowed over each other 
and congealed ("/)ttAo^/«)f?"*-). These, and many other fantastic coiled 
shapes were exhibited by the Vesuvian lava of 1858, and are admirably 
displayed by the peculiarly liquid glassy lavas of Kilauea,- Risalts 
possessing extreme liquidity have flowed for great distances vrith singularly 
smooth surfaces. A large area which has been flooded with lava is 
perhaps the most hideous and appalling scene of desolation anywhere to 
be found on the surface of the globe. 

A lava-stream usually spreads out as it descends from its point of 
escape, and moves more slowly. Its sides look like huge embankments, 

^ Nordenskiold, Oeol. Mag. 2nd dec. iii. p. 292. G. vom Rath, Monatsber. K. Preuss. 
AkfuL WUs. 1876, i». 282. Xeues Jahrb. 1876, p. 52, &nd postea, p. 338. 

- For descriptions of Vesuvian lava- streams, see the various memoirs and works cited, 
ante^ ]>. 195. For those of Etna, Sartorius von Waltershausen and A. von Lasaulx, * Der 
Aetna,' ii. p. 390. The rugged scoriaceous lava-surfaces are known in Hawaii as aa^ the 
smooth coiled and ropy surfaces are there called jxthoehoe. Dana, 'Characteristics ol 
Volcanoes,' p. 9. The same stream of lava may exhibit both these aspects in different parts 
of its course. Ibid. p. 209 and Mr. Johnston-Lavis' papers on Vesuvius, already cited 
p. 195. 



DVXAMICAL GEOLOGY 



BCXIK III PAST I 



or like some of the long moiiixis of " clinkers " in a great manufacturing 
diatrict. The advancing end is often much steeper, creeping onward like 




Outflow of Lava. — This tippcai-s to l>e immediately due to the 



8ECT. i § 2 LA VA-STREAMS 219 

expansion of the absorbed vapours and gases in the molten rock. Though 
these vapours may reach the surface, and even produce tremendous ex- 
plosions, without an actual outcome of lava, yet so Intimately are vapours 
and lava commingled in the subterranean reservoirs, that they commonly 
rise together, and the explosions of the one lead to the outflow of the 
other. The first point at which the lava makes its appearance at the 
surface will largely depend upon the structure of the ground. Two 
causes have been assigned on a foregoing page (p. 209) for the Assuring 
of a volcanic cone. As the molten mass rises within the chimnev of the 
volcano, continued explosions of vapour take place from its upper siu'face. 
The violence of these may be inferred from the vast clouds of steam, 
ashes, and stones hurled to so great a height into the air, and from the 
concussions of the ground, which may be felt at distances of more than 
1 00 miles from the volcano. It need not be a matter of surprise, there- 
fore, that the sides of a great vent, exposed to shocks of such intensity, 
should at last give way, and that large divergent fissures should be opened 
down the cone. Again, the hydrostatic pressure of the column of lava 
must, at a depth of 1 000 feet below the top of the column, exert a pres- 
sure of between 70 and 80 tons on each square foot of the surrounding walls 
(p. 209). We may well believe that such a force, acting upon the walls of 
a funnel already shattered by a succession of terrific explosions, may 
prove too great for their resistance. WTien this happens, the lava pours 
forth from the outside of the cone. On a much-fissured cone, lava may 
issue freely from many points, so that a volcano so affected has been 
graphically described as " sweating fire.'' 

In a lofty volcano, lava occasionally rises to the lip of the crater and 
flows out there ; but more frequently it escapes from some fissure or ori- 
fice in a weak part of the cone. In minor volcanoes, on the other hand, 
where the explosions are less violent, and where the thickness of the 
cone in proportion to the diameter of the funnel is often greater, the 
lava very commonly rises into the crater. Should the crater-walls be 
too weak to resist the pressure of the molten mass, they give way, and 
the lava rushes out from the breach. This is seen to have happened in 
several of the puys of Auvergne, so well figured and described by Scrope 
(Fig. 48).^ But if the crater be massive enough to withstand the pressure, 
the lava may at last flow out from the lowest part of the rim. 

In a tall column of molten lava, there mav be a variation in the 
density of its different parts, the heaviest naturally gravitating to the 
bottom. It has been observed by Ch. V^lain that at the Isle of Bourbon 
(Reunion), the lavas escaping from the l>ase of the volcanic cone are 
denser and more basic than those which flow out from the lip of the crater. - 

* For descriptions of tliis region, see Scrope's * Geology and Extinct Volcanoes 
of Central France,' 2d edit. 1858. H. Lecoq's 'Epoqiies geologiqnes de I'Auverjrne.' 
1867. Michel Levy, BuH. S,ir. ijdi. France, xviii. (1890), p. 688. The succession of 
volcanic rocks in Velay is described by M. Boule, Bull. *S(h: Oeol. France, xviii. (1889), 
p. 174, and in Bull. Carte <r^i>l. tfe la France, Xo. 28 (1892) ; see also op. cif. No. 13 for 
a memoir by P. Termier. 

- *Les Volcans,' j». 36. For references relating to this island, see p. 243. 



220 



tiYXAMKAL HEnLOGY 



As soon AB the molt€n rock reaches the surface, the superheated water- 
vupour or gas, dissolved ^dthiIl its mass, escajjcs copioush', and hangs as a 
dense white cloud over the moving current. The lava-streams of Vesuving 
sometimes appear with as dense a steam-cloud at their lower ends as that 




which escapes at the same time from the main crater. Even after the 
molten mass has flowed several miles, steam continues to rise abundantly 
lioth from its end and from numerous points along its surface, and 
continues to do so for many weeks, months, or it may be for several 
years. 

Should the point of escape of a lava-stream lie well down on the cone, 
far below the summit of the lava-column in the funnel, the molten rock, 
on its first escape, driven by hydrostatic pressure, will sometimes spout 
up high into the air — a fountain of molten rock. This was observed in 
1794 on Vesuvius, and in 1^32 on Etna. In the eruption of 1852 at 
Mauna Loa, an unbroken fountain of lava, from 200 to 700 feet in height 
and 1000 feet broad, burst out at the base of the cone. Similar " geysers " 
of molten rock have subsequently been noticed in the same region. Thus 
in March and April 186)*, four fiery fountains, throwing lava to heights 
varying from .")00 to 1000 feet, continued to play for several weeks. 
According to Mr. Coan, such outbursts take place from the bottom of a 
column of lava 3000 feet high. The volcano of Mauna Loa strikingly 
illustrates another feature of volcanic dynamics in the position and out- 
flow of lava. It bears u|x>n its Hanks at a distance of 20 miles, but 
10,000 feet lower, the huge crater Kilauea. As Dana has pointed out, 
these orifices form ])art of one mountain, yet the column of lava stajids 
10,000 feet higher in one conduit than in the other. On a far smaller 
scale the same independence occurs among the several pipes of some of 
the geysers in the Yellowstone region of North America. 

From the wide extent of basalt-dykes, such as those of Tertiary age 
in Britain, which rise to the surface at a distance of 200 miles from the 
main remnants of the volcanic outbursts of their time, and are found over 
an ai'ca of jierhaps 100,000 square miles, it is evident that molten lava 



SECT, i 5; i LA VA -ST RE A MS 2 2 1 

may sometimes occupy a far greater space within the crust than might be 
inferred from the dimensions and outpourings even of the largest volcanic 
cone. There can be no doubt that vast reservoirs of melted rock, impreg- 
nated with superheated vapours, must foimerly have existed, if they do 
not exist still, beneath extensive tracts of country (p. 583). Yet even 
in these more stupendous manifestations of volcanism, the lava should be 
regarded rather as the sign than as the cause of volcanic action. The 
cause of the ascent of the lava in volcanic pipes is still obscure : it may 
possibly be due to the compression arising from the secular contraction 
of the earth. But it is doubtless the pressure of the imprisoned vapour, 
and its struggles to get free, which produce the subterranean earthquakes 
and the explosions from the vents. As soon as the vapoiu* finds relief, 
the terrestrial commotion calms down again, until another accumulation of 
vapour demands a repetition of the same phenomena. 

Rate of flow of Lava. — The rate of movement is regulated by the 
fluidity of the lava, by its volume, and by the form and inclination of 
the ground. Hence, as a rule, a lava-stream moves faster at first than 
afterwards, because it has not had time to stiffen, and its slope of descent 
is usually steeper than farther down the mountain. One of the most 
fluid and swiftly flowing lava-streams ever observed on Vesuvius >vas 
that erupted on 1 2th August 1 805. It is said to have rushed down a 
space of 3 Italian (3f English) miles in the first four minutes, but to 
have widened out and moved more slowly as it descended, yet finally to 
have reached Torre del Greco in three hours. A lava erupted by Mauna 
Loa in 1852 went as fast as an ordinary stage-coach, or fifteen miles in 
two hours ; but some of the lavas from that mountain have in parts of 
their course moved with double that rapidity. Long after a current has 
been deeply crusted over with slags and rough slabs of lava, it may con- 
tinue to creep slowly forward for weeks or even months. 

It happens sometimes that, as the lava moves along, the still molten 
mass inside bursts through the outer hardened and deeply seamed crust, 
and rushes out with, at first, a motion much more rapid than that of the 
main stream. Any sudden change in the form or slope of the ground 
affects the flow of the lava. Thus, reaching the edge of a steep defile or 
cliff, the molten rock pours over in a catiiract of glowing, molten rock, 
with clouds of steam, showers of fragments, and a noise utterly indescril>- 
able. Or, on the other hand, encountering a ridge or hill across its 
path, it accumulates until it either finds egress round the side or actually 
overrides and entombs the obstacle. The hardened crust or shell, within 
which the still fluid lava moves, serves to keep the mass from spreading. 
Here and there, inside this crust, the lava subsides, leaving cavernous 
spaces and tunnels into which, when the whole is cold, one may creep, 
and which are sometimes festooned with stalactites of lava (p. 227). 

Size of Lava-streams. — In some cases, lava escaping from craters 
or fissures comes to rest before reaching the base of the slopes, like the 
obsidian current which has congealed on the side of the little volcanic 
island of Volcano. ^ In other instances, the molten rock not only reaches 

* Recent eruptions iu this island have consisted entirely of ashes. A. Baltzer, ZeUsch. 



222 JJYNAMICAL GEOLOdY book iii part i 



the plains but flows for many miles away from the point of eruption. 
Sartorius von Waltershausen computed the lava emitted by Etna in 1865 
at 92 millions of cubic metres, that of 1852 at -420 millions, that of 1669 
at 980 millions, and that of a prc-historic lava-stream near Randazzo at 
more than 1000 millions.^ The most stupendous outpouring of lava on 
record was that which took place in Iceland in the year 1 783. Successive 
streams issued from a fissure about 1 2 miles long, filling up river-gorges 
which were sometimes 600 feet deep and 200 feet broad, and advancing 
inU) the alluvial plains in lakes of molten rock 12 to 15 miles wide and 
100 feet deep. Two currents of lava which, filling up the valley of 
the Skapta, escaped in nearly opposite directions, extended for \b and 
50 miles respectively, their usual thickness being 100 feet. Bischof 
estimated that the total amoiuit of lava poured forth during this single 
eruption " surjmssed in magnitude the bulk of Mont Blanc." ^ 

Varying liquidity of Lava. — All lava, at the time of its expulsion, is 
in a molten condition. It usually consists of a glassy magma in which, by 
reason of the high temjMjrature, most or even all of the mineral constituents 
exist dissolved. Considerable differences, however, have been observed in 
the degree of liquidity. Humboldt and Scrope long ago called attention 
to the thick, short, lumpy forms presented by masses of solidified trachytic 
rocks, which are lighter and more siliceous, and to the thin, widely ex- 
tended sheets assumed by l>asalts, which are heavy and contain much 
iron and Imsic silicates.** It may be inferred that, as a rule, the basalts 
or basic lavas have been more liquid than the trachytes or siliceous lavas. 
The cause of this difference has been variously explained. It may depend 
pirtly uj)on chemical composition, the siliceous being naturally less fusible 
than the basic rocks. But as great differences of fluidity are observable 
even among lavas having nearly the same composition, there would seem 
to bo some further cause for the diversity. Keyer has ingeniously 
maintained that we must look to original differences in the extent to 
which the subterranean igneous magma that supplied the lava has been 
satiu*ated with vapours and gases. Molten rock highly impregnated gives 
rise, he holds, to fragmentary discharges, while when feebly impregnated 
it flows out trampiilly.* On the other hand, Captain C. E. Dutton, who 
has studied the volcanic phenomena of Western Americji and Hawaii, 
suggests that the different degrees of liquidity may dej>end not only on 
chemical differences, but on variations of temperature. He supposes 
that the basaltic lavas which have spread so far in thin sheets, and 
which must have had a comparatively great liquidity, flowed at tem- 
peratures far above that of their melting point, and were, to use his 
phrase, " superfused.*' ^ 

heutsch. (real. Gci. xxvi. (1875), p. 36. G. M«rcalli, *Le Eruzioiii dell' Isola Vulcano/ Rat- 
scfjna XazionalCt 1889 ; also a i>aper by same author iu Atti. N»c. /tal. Sci. Xat,, vol. xxxi. 

1 'Der Aetna,' ii. 393. 

- Lyell, 'Principles,' ii. p. 49. Ilellaud, * Lakis-kratere,* cited rtw^f, p. 202. 

•* Scrope, 'Considerations on Volcanoes* (IS'i.*)), p. 93. 

* ' lieitrag zur Physik der Erui>tionen,' p. 77. 

* " High Plateaux of Utah," (wcntj, and O'eol. »Sttr. Territories. Washington, 1880, chap. v. 




8BCT L g 2 LAVA-STREAMS 223 

The varying degrees of liquidity are manifested in a characterietic way 
on the surface of lava. Thus, in the great lava-pools of Hawaii, the rock 
exhibits a remarkable liquidity, throwing up fountains of molten rock 
to a height of 300 feet or more. During its ebullition in the crater-pools, 
jets and dribbleU, a quarter of an inch in diameter, are tossed up, and 
falling back on one another, make " a column 
of hardened tears of lava," one of which 
(Fig. 49) was found to have attained a height 
of 40 feet, while in other places, the jets 
thrown up and blown aside by the wind give 
me to long threads of glass which He thickly 
together like mown grass, and are known by 
the natives under the name of " Pele's Hair," 
after one of their divinitiea.i Yet although 
the ebullition is caused by the uprise and 
escape of highly heated vapours, there is no 
cloud over the boiling lake itself, heavy white Rg. « 

vapour only escaping at different points J"* <>' "i'"^ l*v«, cnii«r .>f 
along the edge. ''"'"" *'^"'- 

On the other hand, the lavas of Vesuvius and of most modern 
volcanoes, which issue so saturated with vapour as to be nearly concealed 
from view in a cloud of steam, are accompanied by abundant explosions 
of fragmentary materials. Slags and clinkers, torn by explosions of 
steam from the molten rock, are strewn abundantly over the cone, while 
the surface of the lava ia likewise rugged with similar clinkers, which 
may now and then be observed piled up round some more energetic 
steam- spiracle. Sometimes the vapour forces up the lava round such a 
spiracle or fumarole and gradually piles up a rugged column several feet 
or yards in height, as has been observed on Vesuvius* (Figs. 46, 49, 
50). So vast an amount of steam rushes out from one of these orifices, 
and with such boiling and explosion, that the cone of bombs, slags, 
and irregular lumps of lava forms a miniature or parasitic volcano, 
which will remain as a marked cone on its parent mountain long 
after the eruption which gave it birth has ceased. The laia of the 
eruption at Santorin in 1866-67 at first welled out tranquilly, but after 
a few days its outflow was accompanied by explosions and discharges 
of incandescent fragments, which increased until they had covered the 
lava dome with ejected scori*, and had opened a number of crateriform 
mouths on ita summit.' 

There can be no doubt, as above remarked, that the condition of 
liquidity of the tava has in some measure determined the form of the 
eruptions. In one case, there are quiet ouUwellings of the more liquid 
lavas, as at Hawaii ; in another, there are explosive discharges and 

' Dana, Otol. i'.S. Rrjilor. £rj^l., "Geologjr," j). I7S ; ' Chancttristici of Voloiiioes,' 
p. ISO 

- Some 8°^ einniples were observed on this nioantain in the auiunier of 1S9I by Mr. 
Johnrfon-Lsvis, BHI J«.f. 1891. s«[. C. 

' Ponqu^, 'Santorin,' p. xv. 



2i4 



iiYXAMI<AL iiE<iLO»n 



BOUK 111 I'Atlll 



ciiKler-tone^ accomiMiiyiiig the more viscid lavas, as at moat modeni 
voltaiioes. The fonuer has been the condition favourable to the most 
colonsal mitiwiii-ings of molten i-ock, as we see in the basal C-plat«aux of 
Biitain, Faroe, (ireoiilaiid, Idaho, and Ore^ii, the Ghaut«, Abysainia, etc. 
This subject is again referred to at p. :?">5. 




glass 



ivlui (AMdi^ 



rvstallizatioii of Lava. — I'uiiriiig forth with a liquidity like that 
olteii iron, lava sjwedily assianes a more viscous condition and a 
;[■ niDtiiin. Obsidian and other vitreous iiK-ks haie consolidated as 
: yet that they are not ahvays extremely fluid is indicated by the 
t of the obsidian stn<am half-way <lowii the steep northern Bloi>e of 
Voleanii. Even in such jK-rfevt iiiiturid glass as obsidian, microscojac 
crystallites and crystals arc uftnally present, and in prodigious numbers 
(pp. 11">, Ifi'J). Ill most lavas, deWtrification has proceeded so far before 
the final stitt'enin;:, that the original glassy magma lias passed into a more 
or less completely lithoid or crystalline mass. 

That lava may possess an appreciably crystalline structure while still 
in motion, has often been proved at Vesuvius, where well-defined crystals 
of the infusible luucite may be observecl in a molten magma of the other 
minerals, jxirtions of the white-hot rock in this condition being ladled out, 



.SECT, i § 2 LA VA'STREAMS 225 

impressed with a stamp and suddenly congealed. The fluxion-structure 
above described (pp. 100, 120) furnishes interesting evidence of this fact 
in many ancient as well as modern lavas. 

There is reason to believe that in the molten magma beneath a volcano 
considerable progress may be made in the development of some crystal- 
line minerals out of the surrounding glass, and that this crystalline portion 
may be to some extent separated from the vitreous residue. Hence where 
this has taken place, subsequent eruptions may give rise to a more crystal- 
line and probably more basic lava from one point of emission and a more 
glassy and probably more acid lava from another vent Or we may con- 
ceive that the two portions of the magma may be subsequently mingled 
again in various proportions before eruption. ^ If the process of differ- 
entiation should continue, as seems, natural, during the lapse of a whole 
cycle of a volcano's history, the earlier lavas would be more basic than the 
later. 

The crystalline structure of lava has been supposed to be in some 
measure determined by the presence of the volcanic vapours and gases 
with which the molten rock is impregnated, the rapid escape of these 
vapours preventing the formation of the crystalline structure, and leaving 
the lava in the condition of a more or less perfect glass. But the experi- 
ments of MM. Fouqu6 and Michel- L^vy (postea^ p 302) have shown 
that rocks, having in every essential particular the characters of volcanic 
lavas, may be artificially produced under ordinary atmospheric pressiu'e by 
simple dry fusion. There appears to be no doubt that the presence of 
water lowers the fusion-point of silicates, though what precise influence 
the dissolved vapours exert upon the ultimate consolidation of molten 
lava has yet to be ascertained. Difference in the rate of cooling has 
doubtless been an important, if not the main, factor in determining the 
various conditions of texture of lava-streams. The crystalline stnicture 
may be expected to be most perfect where, as within thick masses of rock, 
the cooling has been prolonged, and where, consequently, the crystals have 
had ample time and opportunity for their formation. On the other 
hand, the glassy structure will naturally be most perfectly shown where 
the cooling has been most rapid, as in the vitreous crust on the walls of 
dykes already referred to (pp. 171, 210). Eocks crystallizing in the 
deeper parts of a volcano usually possess a more coarsely crystalline 
structure than those which crystallize at or near to the surface (p. 564). 

Temperature of Lava. — It would be of the highest interest and 
importance to know accurately the temperature at which a lava-stream 
first issues. Measurements not altogether satisfactory have been taken 
at various distances below the point of emission, where the moving lava 
could be safely approached. Experiments made at Vesuvius by Scacchi 
And Sainte-Claire Deville in 1855, by thrusting thin wires of silver, iron, 
and copper into the lava, indicated a temperature of scarcely 700° C. 
«(1228* Fahr.) Observations of a similar kind, made in 1819, when a 
diver wire ^(^th inch in diameter at once melted in the Vesuvian lava of 

' Compare the observation of Ch. Velain cited ante, p. 219, and the remarks jpoj^/ea, pp. 
J262, 269, 564. Consult on this subject a paper by Prof. Judd, Oeol. Mag. 1888, p. 1. 

Q 



226 DYNAMICAL GEOLOGY book m part i 



that year, gave a greatly higher temperature, the melting-point of silver 
being about 1 800^ Fahr. But copper wire has also been melted, the point 
of fusion of this metal being about 2204"" Fahr. Evidence of the hi^ 
temperatiu'e of lava has likewise been adduced from the alteration it has 
effected upon refractory substances in its progress, as where, at Torre del 
Greco, it overflowed the houses, and was after^vards found to have fused 
the fine edges of flints, to have decomposed brass into its component 
metals, the copper actually crystallizing, and to have melted silver, and 
even sublimed it into small octahedral crystals (p. 230). The lava of 
Santorin has caught up pieces of limestone, and has formed out of them 
nodules containing crystallized anorthite, augite, sphene, black garnet^ 
and particularly wollastonite.^ The initial temj^erature of lava, as it fint 
issues from the Vesuvian fimnel, is probably considerably more than 
2000'' Fahr. Obviously the dissolved water (or water-substance, for, as 
already remarked, the temperatui'e is far above the critical point of wator, 
and its comiK)nent gases may exist dissociated) must possess as high a 
temperatiu'e as that of the white-hot lava in which it is contained. The 
existence of the elements of water at a white heat, even in rocks which 
have reached the surface, is a fact of no little significance in the theoretical 
consideration of hypogene action. 

Inclination and thickness of lava-flows. — It was at one time 
supposed that lava coidd not consolidate in beds on such steep slopes as 
those of most volcanoes. Hence arose the " elevation-crater theory " 
(described at p. 241), in which the inclined position of lavas round a vol- 
canic vent was explained by upheaval after their emission. Observations 
all over the world, however, have now demonstrated that lava, with all 
its characteristic features, can consolidate on slopes of even 35° and 40*.* 
The lava in the Hawaii Islands has cooled rapidly on slopes of 25**, that 
from Vesuvius, in 1855, is here and there as steep as 30**, while the older 
lavas in Monte Somma are sometimes inclined at 45"". On the east side 
of Etna, a cascade of lava, which in 1689 poured into the vast hollow of 
the Cava Grande, has an inclination varying from 18^ to 48"*, with an 
average thickness of 16 feet. On Mauna Loa some lava-flows are said to 
have congealed on slopes of 49", 60°, and even 90",^ though in these 
cases it coidd only be a layer of rock, stiflening and adhering to the surface 
of the dediWtv. On the other hand, lava-streams have travelled consider- 
able distances over ground that to the eye looks quite level. Among the 
Hawaiian islands a declivity of 1^ or less has been quite sufficient for the 
flow of the extremely liquid and mobile lavas of that region. In the 
great lava-fields of the Snake River region of the Western Territories <rf 
the United States the basalts, which must also have been extremely liquid, 
have flowed over slopes of much less than l®."* The breadth and length 
of a lava-stream, as well as the form of its surface, depend mainly upaa 
the liquidity of the molten material at the time of outflow. Even when 

^ Fouque, * Santoriu,' p. 206. 
■"' Lyell on the cousolidation of lava on steep slopes, Phif. Trans. 1858. 
^ J. D. Dana. Amer. Juio-. iki. xxxv. (1888), p. 32. 
* J. D. Dana, ' Characteristics of Volcanoes," p. 12. 



8BCT. i § 2 LA VA-STREAMS 227 



it consolidates on a steep slope, a stream of lava forms a sheet with parallel 
upper and under surfaces, a general uniformity of thickness, and often 
greater evenness of surface, than where the angle of descent is low. The 
thickness varies indefinitely ; many basalts which have been poured out in 
a remarkably liquid condition have solidified in beds not more than 1 
or 1 2 feet thick. On the other hand, more pasty lavas, and lavas which 
have flowed into narrow valleys, may be piled up in solid masses to a 
thickness of several hundred feet (pp. 222, 229). 

Structure of a lava-stream. — Lava-streams are sometimes nearly 
homogeneous throughout. In general, however, they each show three 
component layers. At the bottom lies a rough, slaggy mass, produced by 
the rapid cooling of the lava, and the breaking up and continued onward 
motion of the scorifoim layer. The central and main portion of the 
stream consists of solid lava, often, however, with a more or less carious 
and vesicular texture. The upper part, as we have seen, may be a%nass 
of rough broken-up slabs, scoriae, or clinkers. The proportions borne by 
these respective layers to each other vary continually. Some of the more 
fluid ropy lavas of Vesuvius have an inconstant and thin slaggy crust ; 
others may be said to consist of little else than scorise from top to 
bottom. Throughout the whole mass of a lava -current, but more 
especially along its upper surface, the absorbed or dissolved water-vapour 
expands with diminution of pressure, and pushing the molten rock aside, 
segregates into small bubbles or irregular cavities. Hence, when the lava 
solidifies, these steam -holes are 

seen to be sometimes so abundant " — ^c^r-^ <c2c:^ tcx: 
that a detached portion of the rock 
containing them will float in water 
(pumice). They are often elon- 
gated in the direction of the " "^"^ 

motion of the lava -stream (Fig. FIr. 51.-Elongation of vesicles in direction of flow 

51). Sometimes, indeed, where 

the cells are numerous, their elongation in one direction gives a fissile 

structure to the rock. 

A singular feature in many lava-streams are the tunnels and caverns 
already referred to (p. 221) as obsei-vable in them. These cavities have 
doubtless arisen during the flow of the mass when the upper and under 
portions had solidified and were creeping sluggishly onward, while the still 
molten interior was able to move faster and thus to leave empty spaces 
behind it. Such tunnels may frequently be seen among the Vesuvian lava- 
streams. Some remarkable examples are described from the highly glassy 
lavas of Hawaii, where they are sometimes from 2 to 10 feet in height 
and 30 feet broad, but with large lateral expansions. The walls of these 
Hawaiian lava-chambers are smooth and even glassy, and from their roofs 
hang slender stalactites of lava 20 to 30 inches long, while on the floor 
below little mounds of lava-stalagmite have formed. The precise mode 
of origin of these curious appendages is not yet understood.^ 

In passing from a fluid to a solid condition, and thus contracting, 

^ See Dana's * Characteristics of Volcanoes,* pp. 209, 33'2. 




228 DYNAMICAL GEOLOGY book ra parti 

lava acquires different sti-uctures. Lines of divisional planes or 
joints traverse it, es]>ecially ])eqx*ndicular to the upi)er and under 
surfaces of the sheet These sometimes assume prismatic forms, 
dividing the rock into columns, as is so frequently to be observed in 
basalt. They are described in Book IV. Part IL, together with other 
forms of joints. 

Vapours and sublimations of a lava-stream. — Besides steam, 
many other vaix)urs, absorbed in the original subterranean molten magma, 
escape from the fissures of a lava-stream. Such vapours are copiously 
disengaged at fumarftlfs (pp. 194, 195). Among the exhalations^ 
chlorides abound, particularly chloride of sodium, which appears, not 
only in fissures, but even over the cooled crust of the lava, in 
small crystals, in tufts, or as a granidar and even glassy incrustation. 
Chloride of iron is de}K)sited as a yellow coating at fumaroles, where also 
briglft emerald-gi*een films and scales of chloride of copper may be more 
rarely observied. Many chemical changes take place in the escape of 
these vapours. Thus sj^ecular-iron, either the result of the mutual decom- 
}X)sition of steam and iron-chloride, or of the oxidation of magnetite, 
forms abundant scales, plates, and small crystals in the fumaroles and 
vesicles of some lavas. Sal-ammoniac also appears in large quantity on 
many lavas, not merely in the fissures, but also on the upper surface. In 
these cases, it is not directly a volcanic product, but results from acme 
decomposition, possibly from the gases evolved by the sudden destruction 
of vegetation. It has, however, been observed also in the crater of Etna, 
where the co-operation of organic substance is hanlly conceivable, and 
where perhaps it may arise from the decomposition of aqueous vapour, 
whereby a combination is formed with atmosphenc nitrogen. Sulphur, 
breislakite, szaboite, tenorite, alum, sulphates of iron, soda and potash, 
and other minerals are also found. 

Slow cooling of lava. — The hardened crust of a lava-stream 
is a bad conductor of heat. Consequently, the surface of the stream 
may have become cool enough to be walked upon, though the red- 
hot mass may be observed through the rents to lie only a few inches 
IkjIow. Many years, therefore, may elapse before the temperature of 
the whole mass has fallen to that of the siu*rounding soil Eleven 
months after an eruption of Etna, Spallanzani could see that the 
lava was still red-hot at the bottom of the fissures, and a stick thrust 
into one of them instantly took fire. The Vesiivian lava of 1785 was 
found by Breislak, seven years afterwards, to be still hot and steaming 
internally, though lichens had already taken root on its surface. The 
ropy lava enipted by Vesuvius in 1858 was observed by the author in 
1870 to be still so hot, even near its termination, that steam issued 
abundantly from its rents, many of which were too warm to allow the 
hand to be held in them, and three years later it was still steaming 
abundantly. Hoffmann records that from the lava which flowed from 
Etna in 1787, steam was still issuing in 1830. Yet more remarkable 
is the case of Jorullo, in Mexico, which sent out lava in 1759. Twenty- 
one years later a cigar could be lighted at its fissures ; after 44 years it 



SECT, i § 2 LAVA-STREAMS 229 



was still visibly steaming; and even in 1846, that is, after 87 years of 
cooling, two vapour-columns were still rising from it.^ 

This extremely slow rate of cooling has justly been regarded as a 
point of high geological significance, in regard to the secular cooling and 
probable internal temperature of our globe. Some geologists have 
argued, indeed, that if so comparatively small a portion of molten matter 
as a lava-stream can maintain a high temperature under a thin, cold crust 
for so many years, we may, from analogy, feel little hesitation in believ- 
ing that the enormously vaster mass of the globe may, beneath a relatively 
thin crust, still continue in a molten condition within. More legitimate 
deductions, however, might be drawn from mor^ accurate and precise 
measurements of the rate of loss of heat, and of its Variations in different 
lava -streams. Lord Kelvin, for instance, has suggested that, by 
measuring the temperature of intrusive masses of igneous rock in 
coal-workings and elsewhere, and comparing it with that of other non- 
volcanic rocks in the same regions, we might obtain data for calculating 
the time which has elapsed since these igneous sheets were enipted 
(aniej p. 50). 

Effects of lava-streams on superficial waters and topo- 
graphy. — In its descent, a stream of lata may reach a water-course, and, 
by throwing itself as an embankment across the stream, may pond back 
the water and form a lake. Such is the origin of the picturesque Lake 
Aidat in Auvergne. Or the molten current may usurp the channel V)f 
the stream, and completely bury the whole valley, as has happened again 
and again among the vast lava-fields of Iceland. Few changes in physio- 
graphy are so rapid and so enduring as this. The channel which has 
required, doubtless, many thousands of years for the water laboriously 
to excavate, is sealed up in a few houi-s under 100 feet or more of stone, 
and another vastly protracted interval may elapse before this newer pile 
is similarly eroded.*-^ 

By suddenly overflowing a brook or pool of water, molten lava some- 
times has its outer crust shattered to fragments by a sharp explosion of 
the generated steam, while the fluid mass within rushes out on all sides.** 
The lava emitted by Mauna Loa, Hawaii, in the spring of 1868 flowed 
out to sea, and added half a mile to the extent of the island at that point. 
At the end of the stream three cinder-cones formed from the contact of 
the lava with the water, and Captain Dutton calls special attention to 
the fact that not only in this instance, but in other examples among the 
Hawaiian lavas which have reached the sea, there is clear evidence of the 
formation of ordinary volcanic craters by the accidental contact of lava 
with water."* The lavas of Etna and Vesuvius have also protruded into 

' E. Schleiden, quoted by Naumanu, * Geognosie,' i. p. 1 60. 

* For an example of tlie conversion of a lava-buried river-bed into a hill- top by long- 
continued denudation, see Quart. Journ. Oeol. Six;. 1871, p. 803. 

' Explosions of tliis nature have been observed on Etna, where tlie lava has suddenly 
come in contact with water or snow, considerable loss of life being sometimes the result. 
Sartorins von Waltershausen and A. von Lasaulx, * I)er Aetna,* i. pp. 295, 300. 

< U.S. Qeol, Report for 1882-83, p. 181. 



230 DYNAMICAL GEOLOGY book m part i 

the sea, but, owing probably to their more viscous and lithoid oonditicm 
and lower temperature, they do not seem to have given rise to exploeiye 
action at their seaward ends. Thus a current from the latter mountain 
entered the Mediterranean at Torre del Greco in 1794, and pushed its 
way for 360 feet outwards, with a breadth of 1100 and a height of 16 
feet. So quietly did it advance, that Breislak could sail round it in a 
boat and observe its progi'ess. 

By the outpoiuing of lava, two important kinds of geological change 
are produced. (1) Stream-courses, lakes, ravines, valleys, in shorty aO 
the minor features of a landscape, may be completely overwhelmed under 
a thick sheet of lava. The drainage of the district being thus effectually 
altered, the numerous changes which flow from the operations of running 
water over the land are arrested and made to begin again in new channek 
(2) Considerable alterations may likewise be caused by the effects of the 
heat and vapoiurs of the lava upon the subjacent or contiguous ground. 
Instances have Ijeen observed in which the lava has actually melted 
down opposing rocks, or masses of slags on its own surface. Interesting 
observations, already referred to (p. 226), have been made at Torre del 
Greco under the lava-stream which overflowed part of that town in 179i. 
It was foimd that the window-panes of the houses had been devitrified 
into a white, translucent, stony substance ; that pieces of limestone had 
acquired an open, sandy, granular texture, without loss of carbon-dioxide, 
and that iron, brass, lead, copj)er, and silver objects had been greatly 
altered, some of the metals being actually su])limed. We can understand, 
therefore, that, retaining its heat for so long a time, a mass of lava may 
induce many crystalline structures, rearrangements, or decompositions in 
the rocks over which it comes to rest, and proceeds slowly to cool. This is a 
(fuestion of considerable importance in relation to the behaviour of ancient 
lavas which after having been intnided among rocks beneath the surface, 
have subsefjuently been exposed by denudation (Book IV. Part VII.) 

But on the other hand, the exceedingly trifling change produced, even 
by a massive sheet of lava, has often been remarked with astonishment 
On the flank of Vesuvius, vines and trees may be seen still flourishing 
on little islets of the older land-surface, completely surrounded by a flood 
of lava. Dana has given an instructive account of the descent of a laTa- 
stream from Kilauea in June 1840. Islet-like spaces of forest were left 
in the midst of the lava, many of the trees being still alive. Where the 
lava flowed round the trees, the stumps were usually consumed, and 
cylindrical holes or casts remained in the lava, either empty or filled 
with charcoal In many cases, the fallen crown of the tree lay near, and 
so little damaged that the epiphytic plants on it began to grow again. 
Yet so fluid was the lava that it himg in pendent stalactites from the 
branches, which nevertheless, though clasped round by the molten rock, 
had barely their bark scorched. Again, for nearly 100 years there has 
lain on the flank of Etna a large sheet of ice, which, originally in the 
form of a thick mass of snow% was ovci'flowed by lava, and has thereby 
been protected from the evaporation and thaw which would certainly 
have dissipated it long ago, had it been exposed to the air. The heat oi 



8ECT. i § 2 ELEVATION AND SUBSIDENCE AT VOLCANOES 231 

the lava has not sufficed to melt it. Extensive tracts of snow were likewise 
overspread by lava from the same mountain in 1879. In other cases, 
snow and ice have been melted in large quantities by overflowing lava. 
The great floods of water which rushed down the flank of Etna, after an 
eruption of the mouuDain in the spring of 1755, and similar deluges at 
Cotopaxi, are thus explained. 

One further aspect of a lava-stream may be noticed here — the effect 
of time upon its surface. While all kinds of lava must, in the end, 
crumble down under the influence of atmospheric waste and, where other 
conditions permit, become coated with soil, and support some kind of 
vegetation, yet extraordinary differences may be observed in the facility 
with which different lavarstreams yield to this change, even on the flank 
of the same mountain. Every one who ascends the slopes of Vesuvius 
remarks this fact. After a little practice, it is not difficult there to trace 
the limits of certain lavas even from a distance, in some cases by their 
verdure, in others by their barrenness. Five hundred years have not 
sufficed to clothe with green the still naked surface of the Catanian lava 
of 1381 ; while some of the lavas of the present century have long given 
footing to bushes of furze. ^ Some of the younger lavas of Auvergne, 
which certainly flowed in times anterior to those of history, are still 
singularly bare and rugged. Yet, on the whole, where lava is directly 
exposed to the atmosphere, without receiving protection from occasional 
showers of volcanic ash, or where liable to be washed bare by heavy 
torrents of rain, its surface decays in a few years sufficiently to afford 
soil for stray plants in the crevices. When these have taken root they 
help to increase the disintegration ; at last, as the rock is overspread, 
the traces of its volcanic origin fade away from its surface. Some of the 
Vesuvian lavas of the present century already support vineyards. 

Elevation and Subsidence. — Proofs of elevation are frequent among 
volcanic vents which, lying near the sea and containing marine sediments 
among their older erupted materials, supply, in the enclosed marine 
organisms, evidence of the movement. In this way, it is known that 
Etna, Vesuvius, and other Mediterranean volcanoes, began their history 
as submarine vents, and that they owe their present dimensions not only 
to the accumulation of ejected materials, but also, to some extent, to an 
elevation of the sea-bottom. 

Proof of subsidence is less easily traced, but indications have 
been observed of a sinking of the ground beneath a volcanic vent. 
During the eruption of Santorin in 1866-67, very decided but extremely 
local subsidence took place near the vent in the centre of the old 
crater. The discharge of such prodigious quantities of material 
may tend to produce cavernous spaces in the terrestrial crust, and the 
weight of the ejected lavas and tuffs may still further contribute to a 
general settlement of the gi'ound around a volcanic focus. 

If we consider the records of volcanic action in past geological time we 
meet with many proofs that it took place in areas where the predominant 
terrestrial movement was one of subsidence. Thus among the Palaeozoic 

' On the weathering of the Etna lavas, see * Der Aetna,* ii. p. 397. 



232 DYNAMICAL GEOLOGY book m part i 



systems of Britain the Cambrian, Siliuian, Devonian, Carboniferous, and 
Permian volcanoes successively appeared, and their lavas and tuflfs were 
carried down and buried under thousands of feet of sedimentary 
deposits.^ 

Torrents of Water and Mud. — We have seen that large quantities 
of water accompany many volcanic eruptions. In some cases, where 
ancient crater-lakes or internal reservoirs, shaken by repeated detonations, 
have been finally dismpted, the mud which has thereby been liberated 
has issued from the mountain. Such "mud-lava'' (lava d'acqua), on 
account of its li(|uidity and swiftness of motion, is more dreaded for 
destructiveness than even the true melted lavas. On the other hand, 
rain or melted snow or ice, rushing down the cone and taking up loose 
volcanic dust, is converted into a kind of mud that grows more and more 
l)asty as it descends. The mere sudden rush of such large bodies of 
water down the steej) declivity of a volcanic cone cannot fail to effect 
much geological change. Deep trenches are cut out of the loose volcanic 
slopes, and sometimes largo areas of woodland are swept away, the debris 
being strewn over the plains below. 

One of these mud- lavas invaded Herculaneum during the great 
eniption of 79, and by quickly enveloping the houses and their contents, 
has preserved for us so many precious and i>eri8hable monuments of 
antiquity. In the same district, during the eruption of 1622, a torrent 
of this kind j)oured down ui)on the villages of Ottajano and Massa, over- 
throwing walls, filling up streets, and even burying houses with their 
inhabitants. Diuing the gi*eat eruj)tion of Cotoj>axi, in June 1877, 
enonnoiis torrents of water and nuid, produced by the melting of the 
snow and ice of the cone, rushed down from the mountiiin. Huge portions 
of the glaciers of the mountain were detached by the heat of the rocks 
below them and rushed down Inxlily breaking up inU) blocks. The villages 
all roiuid the mountain to a distiince of sometimes more than ten geo- 
graphical miles were left deeply buried under a deposit of mud mixed 
with blocks of lava, ashes, pieces of wood, lumps of ice, &c.- Many of 
the volcanoes of Central and South America discharge large quantities of 
mud directly from their craters. Thus, in the year 1691, Imliaburu, one 
of the Andes of Quito, emitted floods of mud so largely charged with 
dead fish that ])estilential fevers arose from the subsequent effluvia. 
Seven years later (1698), during an explosion of another of the same 
range of lofty mountains, Carguairazo (14,706 feet), the summit of the 
cone is said to have fallen in, while torrents of mud containing immense 
numbers of the flsh PipnehHlus (/f/clojmm, jx)ured forth and covered the 
ground over a sjmce of four square leagues. The carbonaceous mud 
(locally called mot/a) emitted by the Quito volcanoes sometimes escapes 
from lateral fissures, sometimes from the cratere. Its organic contents, 
and notably its siluroid fish, which are the same as those found living 
in the streams above ground, prove that the water is derived from the 
surface, and accumulates in craters or undergi-ound cavities until dis- 

* Presidential Addresses, Quart. Jovrn. iieol. ^s>c. xlvii. (1891), xlviii. (1892'r. 

- Wolf. XfMsJohrf,. 1878, p. 133. 



8ECT L § 2 SILLS AND DYKES 233 



charged by volcanic action. Similar but even more stupendous and 
destructive outpourings have taken place from the volcanoes of Java, 
where wide tracts of luxuriant vegetation have at different times been 
buried under masses of dark grey mud, sometimes 100 feet thick, with 
a rough hillocky surface from which the top of a submerged palm-tree 
would here and there protrude. 

Between the destructive effects of mere water- torrents and that of 
these mud-floods there is, of course, the notable difference that, whereas 
in the former case a portion of the surface is swept away, in the latter, 
while sometimes considerable demolition of the surface takes place at first, 
the main result is the burying of the ground under a new tumultuous 
deposit by which the typography is greatly changed, not only as regards 
its temporary aspect, but in its more permanent features, such as the 
position and form of its water-courses. 

Effects of the Closingr of a Volcanic Chimney — Sills and Dykes. — 
A study of the volcanic phenomena of former geological periods, where 
the structure of the interior of volcanoes and their funnels has been laid 
bare by denudation, shows that in many cases a vent becomes plugged up 
by the ascent and consolidation of solid material in it, while yet the 
eruptive energy of the volcano, though lessened, has not ceased. A time 
is reached when the ascending magma, impelled by pressure from below, 
can no longer overcome the resistance of the coliunn of solid lava or com- 
pacted agglomerate which has sealed up the orifice of discharge, or at least 
when it can more easily force a passage for itself between the sedimentary 
strata on which the whole volcanic pile may rest, or between the lava 
sheets at the base of the pile, or into fissui'es in either or both of these 
groups. Hence arise intrusive sheets or sills and dykes or veins (see pp. 
573, 577). That these later manifestations of volcanic energy have some- 
times taken place on a great scale is shown by the number and size of 
the sills which are found at the base of the Palaeozoic volcanic groups of 
Britain. This feature is a remarkably striking feature of the rocks that 
underlie the great Lower Silurian volcanic outflows of Arenig and 
Cader Idris in North Wales. It recurs so frequently, not only among 
Palaeozoic volcanic phenomena but quite as markedly among those of 
Tertiary age in the British Isles, that it must be regarded as marking an 
ordinary phase of volcanic action. But it remains of course invisible until 
in the progress of denudation a volcanic cone is cut down to the roots. 

Exhalations of Vapours and Gases. — A volcano, as its activity 
wanes, may pass into the Solfatara stage, when only volatile emana- 
tions are discharged. The well-known Solfatara near Naples, since 
its. last eruption in 1198, has constantly discharged steam and 
sulphurous vapours. The island of Volcano has now passed also 
into this phase, though giving vent to occasional explosions. Numerous 
other examples occur among the old volcanic tracts of Italy, where 
they have been termed soffioni. Steam, escaping in conspicuous jets, 
sulphiuretted hydrogen, hydrochloric acid, and carbonic acid are 
particularly noticeable at these orifices. The vapours in rising condense. 
The sulphuretted hydrogen partially oxidises into sulphuric acid, which 



234 DYNAMICAL GEOLOGY book hi part i 

powerfully corrodes the surrounding rocks. The lava or tuff through 
which the hot vapours rise is bleached into a white or yellowish crumbling 
clay, in which, however, the less easily corroded crystals may still be 
recognised in situ. At the same time, sublimates of sulphur or of 
chlorides may be foimed, or the sulphuric acid attacking the lime of the 
silicates gives rise to gypsum, which spreads in a network of threads 
and veins through the hot, steaming, and decomposed mass. In this 
way, at the island of Volcano, obsidian is converted into a snow-white, 
dull, clay-stone-like substance, with crystals of sulphur and gypsum in its 
crevices. Silica is likewise deposited from solution at many orifices, and 
coats the altered rock Avith a crust of chalcedony, hyalite, or some form 
of siliceous sinter. As the result of this action, masses of rock are decom- 
posed beloAv the surface, and neAv deposits of alum, sulphur, sulphides of 
iron and copper, &c., are formed alxjve them. Examples have been 
described from Iceland, Li})ari, Hungary, Terceira, Teneriffe, St. Helem^ 
and many other localities.^ The lagoons of Tuscany are basins into which 
the waters from suftioni are discharged, and where a precipitation of their 
dissolved salts takes place. Among the substances thus deposited are 
gypsum, sulphur, silica, and various alkaline salts ; but the most important 
is boracic acid, the extraction of which constitutes a thriving industry. 
In Chili many solfataras occur among extinct volcanoes.- 

Aiiothcr class of gaseous emanations betokens a condition of volcanic 
activity further advanced towards final extinction. In these, the gas 
is carbou-<lioxide, either issuing directly from the rock or bubbling up 
with water which is often quite cold. The old volcanic districts of 
Kurope furnish many examples. Thus on the shores of the Laacher 
See — ^an ancient crater-lake of the Eifel — the gas issues from numerous 
openings called nutffette, round Avhich dead insects, and occasionally 
mice and birds, may be found. In the same region occiu* hundreds of 
springs more or less charged with this gas. The famous Valley of 
Death in Java contains one of the most remarkable gas -springs in 
the Avorld. It is a deep, bosky hollow, from one small space on the 
bottom of which carbon-<lioxide issues so copiously as to form the 
lower stratum of the atmosphere. Tigers, deer, and wild-boar, enticed 
by the shelter of the spot, descend and are si>eedily suffocated. 
Many skeletons, including those of man himself, have been observed. 

As a distinct class of gas-springs, Ave may group and describe here 
the emanations of volatile hydrocarlx)ns, which, when they take fire, 
are known as Fire-wells. These are not of volcanic origin, but arise 
from changes within the solid rocks underneath. They occur in many 
of the districts Avhere mud- volcanoes apj^ear, as in northern Italy, on 
the Caspian, in Mesopjtamia, in southern Kurdistan, and in many parts 
of the United States. It has been observed that they frequently rise 

> Von Buch, *Caiiar. Inseln/ p. 232. Hoffmann, PfHjg. Ann, 1832, pp. 38, 40, 60. 
Hanson, Ann. Ch^m. Pharm. 1847 (Ixii.), p. 10. Darwin, * Volcanic Islands,' p. 29. Hie 
name Propyl ite^ as already mentioned {antr^ p. 169) has been proposed by Rosenbnsch to be 
restricted to certain andesiten and allied rocks alt«re<l by solfataric action. 

- Doineyko, Ann. Mines^ ix, (7® s«'t.) Large numbers of solfataras occur also in loelaad. 



SECT, i § 2 GEYSERS 236 

in regions where beds of rock-salt lie underneath, and as that rock 
has been ascertained often to contain compressed gaseous hydrocarbons, 
the solution of the rock by subterranean water, and the consequent 
liberation of the gas, has been offered as an explanation of these fire- 
wells. 

In the oil regions of Pennsylvania, certain sandy strata occur at 
various geological horizons whence large quantities of petroleum and 
gas are obtained (p. 1 45). In making the borings for oil-wells, reservoirs 
of gas as well as subterranean courses or springs of water are met with. 
When the supply of oil is limited but that of gas is large, a contest for 
possession of the bore-hole sometimes takes place between the gas and 
water. When the machinery is removed and the boring is abandoned, 
the contest is allowed to proceed unimpeded and results in the intermittent 
discharge of columns of water and gas to heights of 1 30 feet or more. 
At night, when the gas has been lighted, the spectacle of one of these 
" fire-geysers " is inconceivably grand.^ 

Geysers. — Eruptive fountains of hot Avater and steam, to which the 
general name of Geysers {i.e. gushers) is given, from the examples in Ice- 
land, which were the first to be seen and described, mark a declining 
phase of volcanic activity. The Great and Little Geysers, the Strokkr 
and other minor springs of hot water in Iceland, have long been celebrated 
examples. More recently another series has been discovered in New 
Zealand. But probably the most remarkable and numerous assemblage 
is that which has been brought to light in the north-west part of the 
territory of Wyoming, and Avhich has been included within the " Yellow- 
stone National Park " — a region set apart by the Congress of the United 
States to be for ever exempt from settlement, and to be retained for the 
instruction of the people. In this singular region the ground in certain 
tracts is honeycombed with passages which communicate with the surface 
by hundreds of openings, whence boiling water and steam are emitted. 
In most cases, the water remains clear, tranquil, and of a deep green- 
blue tint, though many of the otherwise quiet pools are marked 
by patches of rapid ebullition. These pools lie on mounds or sheets of 
sinter, and are usually edged round with a raised rim of the same 
substance, often beautifully fretted and streaked with brilliant 
colours. The eruptive openings usually appear on small, low, 
conical elevations of sinter, from each of which one or more tubular pro- 
jections rise. It is from these irregular tube-like excrescences that the 
eruptions take place. 

The term geyser is restricted to active openings whence coliunns of 
hot water and steam are from time to time ejected ; the non-eruptive 

* Asbburner, Proc. Artier. Phil. *Soc. xvii. (1877), p. 127. StmcdVs PetroUuvi Reporter, 
15th Sept. 1879. Second Oeol. Survey of Pennsylvania, containing Reports by J. Carll, 
1877, 1880. J. S. Newberry, *The First Oil Well.' Harper's Magazine, Oct. 1890. On 
the naphtha districts of the Caspian Sea, Abich, Jdhrb. Oeol. ReicJis. xxix. (1879), p. 165. 
H. Sjogren, op. cit. xxxvii. (1887), p. 47. C. Marvin, * Region of Eternal Fire,' London, 
1884. See also for phenomena in Gallicia, Jahrb. Oeol, Rtichs. xv. pp. 199, 351 ; xviL p. 
291 ; xviii. p. 311 ; xxxi. (1881), p. 131. Proc. Inst. Civ. Engineers, xlii. (1875), p. 843. 



236 DYXAMIOAL GEIiLOGY book in pam i 

pools are only hot springs. A true geyser should thus possess an under- 
ground pipe or passage, terminating at the surface in an opening built 
round with deposits of sinter. At more or less regular intervals, rumblingi 
and sharp <leUiiiations in the pipe are fullon-ed by an agitation of t^e 
water in the basin, and then by the violent e.xpulsion of a, column of water 
and steam to a considerable height in the air. In the Upper Fire Hole 
basin of the Yellowstone Park, one of the geysers, named " Old Faithful " 
(Fig. 52), has ever since the discovery of the region, sent out a column of 
mingled water and steam every sixty-three minutes or thereabouts. The 
column rushes up vtith a loud roar to a height of more than 100 feet, the 
whole eruption not occupying more than about five or six minutes. The 
other geysers of the same district are more capricious in their movements, 




ttBPCc. Fin Hoi* River, 



and some of them more stupendous in the volume of their discharge. 
The eruptions of the Castle, Giant, and Beehive vents are marvellously 
impressive.' 

lu examining the Yellowstone Geyser region in 1679, the author was 
s{>ecially struck by the evident independence of the vents. This was 
shown by thoii- very different levels, as well as by their capricious and 
unsymitathetic ei-uptions. On the same hill-slope, dozens of quiet pools, 
as well as some true geysers, were noticed at dilfcrent levels, from the 
edge of the Fire Hole River up to a height of at least 80 feet above it. 
Yet the lower pools, from which, of course, had there been underground 
1 Sc« Hayd«Q's ReporU for 1S70 and for ISTS. in the latter at which will be (Oand • 
volniiiinous nionognph on the Hot SpringH by A, C. Peale : Cornatock'i Report in Jona'a 
KecuDDaisxance of N'.W. Wyomiug, kr.. I87t. Tlie depmlta of liot Bpringi «i« tartber 
referred to ai. |<|<. 153, 4S3. 



SECT, i § 2 GEYSERS 237 

connection between the different vents, the drainage should have princi- 
pally discharged itself, were often found to be quiet steaming pools 
without outlet, while those at higher points were occasionally in active 
eruption. It seemed also to make no difference in the height or tranquil- 
lity of one of the quietly boiling cauldrons, when an active projection 
of steam and water was going on from a neighbouring vent on the same 
gentle slope. 

Bunsen and Descloiseaux spent some days experimenting at the Ice- 
landic geysers, and ascertained that in the Great Geyser, while the sur- 
face temperature is about 212* Fahr., that of lower portions of the tube 
is much higher — a thermometer giving as high a reading as 266" Fahr. 
The water at a little depth must consequently be 64° above the normal 
boiling-point, but it is kept in the fluid state by the pressure of the over- 
lying column. At the basin, however, the water cools quickly. After an 
explosion it accumulates there, and eventually begins to boil. The 
pressure on the column below being thus relieved, a portion of the super- 
heated water flashes into steam, and as the change passes down the pipe, 
the whole column of water and steam rushes out with great violence. The 
water thereafter gradually collects again in the pipe, and after an 
interval of some hours the operation is renewed. The experiments made 
by Bunsen proved the source of the eruptive action to lie in the hot part 
of the pipe. He hung stones by strings to different depths in the funnel 
of the geyser, and found that only those in the higher part were cast 
out by the rush of water, sometimes to a height of 100 feet, Avhile, at 
the same time, the water at the bottom was hardly disturbed at all. 
These observations give much interest and importance to the phenomena 
of geysers in relation to volcanic action. They show that the eruptive 
force in geysers is steam ; that the water column, even at a comparatively 
small depth, may have a temperature considerably above 212° ; that this 
high temperatiu*e is local ; and that the eruptions of steam and water take 
place periodically, and with such vigour as to eject large stones to a 
height of 100 fect.^ 

The hot water comes up with a considerable percentage of mineral 
matter in solution. According to the analysis of Sandberger, water 
from the Great Geyser of Iceland contains in 10,000 parts the following 
proportions of ingredients : silica, 5097; sodium-carbonate, 1*939; ammo- 
nium-carbonate, 0083; sodium-sulphate, 107; potassium-sulphate, 0*475 ; 
magnesium-sulphate, 042 ; sodium-chloride, 2 5 21 ; sodium-sulphide, 
0-088 ; carbonic acid, 0557 = 11-872.2 

When the water has reached the surface, it deposits the silica as a 

» Comptes Rtndus, xxiii. (1846), p. 934 ; Pogg. AnncU. Ixxu. (1847), p. 159 ; Ixxxiil 
(1851), p. 197. Ann. ChimiCy xxxviii. (1853), pp. 215, 385. The explanation proposed 
for the phenomena observed at the Great Geyser is probably not applicable in 
those cases where the mere local accumulation of steam in suitable reservoirs may be 
snfficient. 

' Annal. Chem, und PharvL 1847, p. 49. A series of detailed analyses of the hot 
spriogs of the Yellowstone National Park will be found in No. 47 of the BuU, U.S. Geol. 
Surv. 1888. 



238 DYXAMICAL GEOLOGY book ill parti 

sinter on the surfaces over which it floAvs or on which it rests.^ The 
deposit, which is not due to mere cooling and evaporation, is curioady 
aided by the presence of liA-ing algie (jxistea, p. 483). It naturally takes 
place fastest along the margins of the pools. Hence the curiously fretted 
rims by which these sheets of water are surrounded, and the tubular or 
cylindrical protulxyrances which rise from the growing domes. Where 
numerous hot springs have issued along a slope, a succession of badm 
gives a curiously picturesque terraced aspect to the ground, as at the 
Manunoth Springs of the Yellowstone Park and at the now destroyed 
terraces of Kotamahana in Ncav Zealand. 

In coiu*se of time, the network of imderground passages undergoei 
alteration. Oiifices that were once active cease to erupt, and even the 
water fails to overflow them. Sinter is no longer formed round them, 
and their siuiaces, exposetl to the weather, crack into fine shaly rubbish 
like commiiuited oyster-shells. Or the cylinder of sinter grows upward 
luitil, by the continued deposit of sinter and the failing force of the 
geyser, the tul>e is finally filled up, and then a dry and crumbling white 
])illar is Ml to mark the site of the extinct geyser. 

Mud- Volcanoes. — ^These are of two kinds : 1st, where the chief source 
of movement is the escajH? of gsiseous discharges ; 2nd, where the actiye 
agent is steam. 

(1) Although not volcanic in the proper sense of the term, certain 
remarkable orifices of eruption may be noticed here, to which the names 
of fnu(il-r*fitvnf'i'.<, i^ilsifs, air-ritlcano€.<y and maccalnbas have been applied 
(Sicily, the Apennines, Caucasus, Kertch, Taman). These are corneal 
hills formeil by the accumidation of fine and usually saline mud, which, 
\nth various gases, is contiiuiously or intermittently given out from the 
onfice or crater in the centre. They occur in groui)s, each hillock being 
sometimes less than a yaiil in height, biu ranging up to elevations of 100 
feet or moi-e. Like true volcanoes, they have their periods of repose, 
when either no dischai'ge takes phice at all, or mud oozes out tranquilly 
from the crater, and their eix>chs of activity, Avhen Islrge volumes of gis, 
and sometimes columns of fiame, rush out with considerable violence and 
explosion, and throw up mud and stones to a height of several hundred 
feet. The gjises play much the s;ime jiart, therefore, in these phenomena 
that steam di^s in those of true volcanoes. They consist of marsh-gss 
and other hydriH*arl>ons, carbon-dioxide, sulphiu-etted hydrogen, and nitro- 
gen, with }x*ti*oleuni A-ajxnu^ The mud is usually cold. In the water 
occur various s;iliiie ingredients, among which common salt generally 
appears *, hence the name, <S({/,'!^vs. Naphtha is likewise frequently 
present. Large piei-es of stone, diifering from those in the neighbonr- 
hood, have been oliserveil among the ejections, indicative doubtless of a 
somewhat deeper soiu*ce than in ortlinary nises. Heav}* rains may wash 
down the minor mud-cones and spread out the material over the ground ; 
but gas-bubbles again appear through the sheet of mud. and by degrees 
a new series of moimds is once more thrown up. 

^ For an accoant of Xht sreyserite of the Tellow>to?itf district, see I^peis by W. tL 
Weed, Am^r. Jo»rn. Sci. xxxrii. (1SS9\ and 9fA .Un. /?./>. U.S. Gf>^. Surr, 1890. 



8BCT. i § 3 STRUCTURE OF VOLCANOES 239 

There can be little doubt that this type of mud-volcano is to be traced 
to chemical changes in progress underneath. Dr. Daubeny explained 
them in Sicily by the slow combustion of beds of sulphur. The 
frequent occurrence of naphtha and of inflammable gas points, in other 
cases, to the disengagement of hydrocarbons from subterranean strata.^ 

(2) The second class of mud-volcano presents itself in true volcanic 
regions, and is due to the escape of hot water and steam through beds of 
tuff or some other friable kind of rock. The mud is kept in ebullition 
by the rise of steam through it. As it become more pasty and the steam 
meets with greater resistance, large bubbles are formed which burst, and 
the more liquid mud from beloAv oozes out from the vent. In this way, 
small cones are built up, many of which have perfect craters atop. In 
the Geyser tracts of the YelloAvstone region, there are instructive examples 
of such active and extinct mud-vents. Some of the extinct cones there 
are not more than a foot high, and might be carefully remoA^ed as museum 
specimens. 

Mud-volcanoes occur in Iceland, Sicily (Maccaluba), in many districts 
of northern Italy, at Tamar and Kertch, at Baku on the Caspian, near 
the mouth of the Indus, and in other parts of the globe.^ 

§ 3. Structure of Volcanoes. 

We have now to consider the manner in which the various solid 
materials ejected by volcanic action are built up at the surface. This 
inquiry ivill be restricted here to the phenomena of modern Aolcanoes, 
including the active and dormant, or recently extinct, phases. Obviously, 
however, in a modern volciino we can study only the upper and external 
portions, the deeper and fundamental parts l>cing still concealed from 
view. But the interior structure has been, in many cases, laid open 
among the volcanic products of ancient vents. As these belong to the 
architecture of the terrestnal crust, they are described in Book IV. The 
student is therefore requested to take the descriptions there given, in 
connection with the foregoing and present sections, as related chaptei*s of 
the study of volcanism. 

Confining attention at present to modern volcanic action, we find that 
the solid materials emitted from the earth's interior are arranged in two 
distinct types of structure, according as the eruptions proceed from large 
central cones or from less prominent vents connected with fissiu'es. In 
the former case, volcanic cones are produced ; in the latter, volcanic 

* The "burning hills " of Turkestan are referred to the subterranean combustion of beds 
of Juraasic Coal. J. Muschketoff, Neues Jahrb. 1876, p. 516. 

' On mud - volcanoes, see Bunsen, Liebujs Annuti/^ Ixiii. (1847), p. 1 ; Abich, Mew. 
Acad. St, Petertburgy 7® ser. t. vi. No. r», ix. No. 4 ; Daubeny's V(tfca.m)c,Sj pp. 264, 539 ; 
Bnist, TV-ans. Bombay Geograph. Sftc. x. p. 154 ; Roberts, Joum. Rot/. Asiatic Snc. 1850 ; 
De Vemeuil, Mem, Soc. Qeol. France, iii. (1838), p. 4 ; Stiflfe, Q. J, Oeol. Sue. xxx. p. 60 ; 
Von Lasaulx, Z. Deutsch. Oeol. Ges. xxx\. p. 457 ; Giimbel, Sitzb. Akad. Miinclt. 1879 ; 
F. R. Mallet, Rec. Oeoi. Surr. Indi<i, xi. ji. 188. H. Sjogren, JaJirb. Geo/. Reichaanst. 
xxxviL (1887), p. 233. 



240 nVXAMICAL GEOLOGY book lii paw i 



plateaux or plains. The tyjje of the volcanic cone, or ordinary volcano, 
is noAv the most abundant and Inist known. 

i. Vcicanic Cones. 

From some weaker jwint of a fissure, or from a vent opened directly 
by explosion, A'olcanic discharges of gas and vapours with their liquid 
and solid accom^xiniments make their wkj to the siu^ace and gradually 
build up a volcanic hill or mountain. Occasionally, eruptions have pro- 
ceeded no further than the first stage of gaseous explosion. A cauldron- 
like cavity has l>een torn open in the ground, and ejected fragments of 
the solid rocks, through which the explosion has emerged, have fall«i 
Ivick into and ix»und the vent Subsequently, after possible subsidence 
of the fragmentary materials in the vent, and even of the sides of the 
orifice, Avater supplieil by rain and filtering from the neighbouring ground 
may pju-tially, or wholly, fill up the cavity, so as to produce a lake either 
with or without a superficial outlet. Under favoiu^ble circumstances, 
vegetation creeping over bare earth and stone may so conceal all evidence 
i\i the origiiuil volcanic action as to make the quiet sheet of water look 
as if it had always 1>een an essential part of the landscape. £3q)lo6ion- 
lakes (Crater-lakes) of this kind occur in districts of extinct volcanoes, as 
in the Eifel (maare\ central Italy, and Auvergne. The craterifonn hollow 
called the Gour de Tazenat, in Vebiv, has a diameter of half a mile and 
lies in the giimite, while another cavity near Confolens, on the left bank 
of the Loire, has also been blown out of the granite and has given passag? 
to no volcanic materials, but only to broken-up granite.^ Other illustra- 
tions in central France are to be found in the I^kes of Pa^in, Mont 
Sinoire, Chauvet, Beurilouse, ChamjHHlaze and Lji Ciodival.- A remarkable 
example is supplioii by the Lonar l^ike in the Indian peninsula, half-way 
between Rnnlxiy and Nag]nir. It lies in the midst of the volcanic plateau 
of the IVccan trai>s, which extend around it for hundreds of miles in 
nearly flat IhhIs that slightly dip aA^iiy from the lake. An almost circular 
dqnvssion, rather more than a mile in diameter, and from 300 to 400 
tVet divp, contains at the Ixittom a shallow lake of Intter saline water, 
de|Kv<iting crystals of tixma (native carlxMiate of soila, the nitnun of the 
ancients). Except to the north and north-east, it is encircled with a 
raisi^l rim of irregularly pileil bUx'ks of liasiilt, identical with that of the 
IkiIs thnnigh which the cavity has Kvn opened. The rim never exceeds 
100 foot, and is often not more th.in 40 or oO feet in height, and cannot 
ixMitain a thousandth (Xirt of the material which once tilled the crater. 
No other evidence of volcanic discharge from this vent is to be seen. 
Some of the contents of the cavity may have been ejected in fine 
p;4rtioles, which have sulisetpiently been removed by denudation ; bat 
it seems more pn.^bable that the existence of the cavity is mainly dae to 
subsidence after the original explosion.' 

- To*.iTT.»iiv, DW Si<. twV'.t*. Fr^.xncf, xivi. 1S^9 . p. 1166; Dunbrvc, Compiet rend, 

* 

- Son>:»e. 'Voloanoe* of iVntral Frano*/ pp. SI. 14:>. 144. 
i^Thu cavi:T miT posdablv mark oue of the v^nts fn>m whicli the htaalt Hoods u 



BsctigS ELEVATION-CRATER THEOBY 241 

In moflt cases, explosions are accompanied by the expulsion of so 
much solid material that a cone gathers round the point of emission. As 
the cone increases in height, by successive additions of ashes or lava to 
its surface, these volcanic sheets are laid down upon progressively steeper 
slopes. The inclination of beds of lava, which must have originally 
issued in a more or less liquid condition, offered formerly a difficulty 
to observers, and suggested the famous theory of Elevation-craters {Er- 
helMnffskratere) of L. von Buch,' £lie de Beaumont,^ and other geologists. 
According to this theory, the conical shape of a volcanic cone arises 
mainly from an upheaval or swelling of the ground, round the vent from 
which the mat«rials are finally expelled. A portion of the earth's crust 
(represented in Fig. 53 as composed of stratified deposits, ab g h) was 
believed to have been pushed up like a huge blister, by forces acting 
from below (at c) until the summit of the dome gave way and volcanic 
materials were emitted. At first these might only partially fill the 




Fig. SS.— SecOon illurtniUve of Uke Eleralion-crater Theory. 

. cavity (as at f), but subsequent eruptions, if sufficiently copious, would 
cover over the truncated edges of the pre-volcanic rocks (as at g ft), and 
would be liable to further upheaval by a renewal of the original upward 
swelling of the site. 

It was a matter of prime importance in the interpretation of volcanic 
action to have this question settled. To Poulett Scrope, Constant 
Prevost, and Lyell, belongs the merit of disproving the Elevation-crater 
theory, Scrope showed conclusively that the steep slope of the lava-beds 
of a volcanic cone was original.^ Constant Provost pointed out that 
there was no more reason why lava should not consolidate on steep slopes 
than that tears or drops of wax should not do so,' Lyell, in successive 
Oq eipltMion ' enters ui<t lakes, 9»s Scrope'a 'Volcaaoes.' Lecoq, 'Epoqnea geologiques 
d« TAnvtrgne,' tome it. ; eompara also VogeUsng, 'Vnleane dec Eifel, ' Bod in Smt» Jahrb, 
1870, pp. 199, 32S, 460. Od Lanar Lake, see Malcolmaon, Tram. Oeol. Soc. 2nd aet. t. 
p. S82. Hedlicott and Blanford'a ' Geo]ogj of India,' p. 379. 

' Pops- '*«•>■ !«, I. "vii, p. 168, 

' Jiifl. Sac atot. FrajKi, iv. p. 357, Ann. da Ml«a, ii. and i. 

' ' Considerationa on Volcanoea,' 1825. Quart. Jonm. Geai. Soc. lil. p. 328. 

* CompUt Rendas, i. (1835), 480; ill. (1855), p. 919. GM. Soc Franct : Mtmiim,^. 
p. 105, and Bail. liv. 217. Social PhiUm. Paris, PnK. Verb. 1843, p. 13. 



BYNAMICAL GEOLOGY 



BOOK III PABT I 



editionB of his works, and subsequently by an examination of the Canarr 
Islands with Hartung, brought forward cogent ai^uments against tlie 
Elevation-crater theory.' A comparison of Fig. 53 with Fig, 54 will show 



iS^-Q.'- 




u bj tlie <lotU<l llu> 



at a glance the difference Ivetwecn thi^ theorj- and the views of volcanic 
atnicture now universally accej)ted. The steep decli^-ities on which lara 
can actually consolidate have licoi) referred to on p. 226. 

The conical form of a volcano in that naturally assumed by a self- 
supporting Tnas^4 of coherent material. It varies slightly accot^ng to 
the nature of the matenals of the cune, the progress of atmoapheric 
denudation, the gmsitioii of the crater, the direction in which materials 
are ejected, the foii;e and dii-ection of the wind diuing an eruption, 
the growth of |Nira»itic cones, and the collaiise due to the dying out of 
volcanic cnerg_v.- 

The cone gi-ows by additions made to its surface during successive 
eruptions, and though liable to great local variation of contour and topo- 
graphy, preserves its general form with singular i>ersistence. Many 
exaggerated pictui-es have l>een drawn of the stccimess of slope in volcanic 
cones, but it is obvious that the angle cannot as a whole exceed the 
maximum inclination of repose of the detrital niatt«r ejected from the 
central chimney.^ A scries of profiles of volcanic cones taken from 
photographs shows how nearly they approach to a common average type.* 
One of the most potent and constant agencies in modifying the out«r 
forms of these eoues is undoubtedly to Ije found in rain and torrents, 
which sweep down the loose detiitus and excavate ravines on the 

' Pkil. Tram. 18.'>S, |i. 703. See the ceraarks of Fouqu.', ' Santorin, ' pp. 400-422. 

= J. Milne, Oe-U. ilaj. 1878. p. 336 ; 1879, p. BOB. Aamnliy. Sue Japan, fr. p. 178. 
G, F. Becker, Amfr. Joiim. S^i. xxx. 1885, p. 283. 

» Cotopaxi a a uotablc example of such <r™)at«rateii represenUtion. Mr. Wbfniper 
totuid that the general anglis o( the nortbeni and souCheni nlope* at ths nma mra ntber 
lewi than 30° ('Travels Auiongat the Great Andei,' |j. 133]. Uuniboldt depicted tht an^e u 
ona of SO* ! 

* See MUne, Sehm. Soc. JapaK, ii., and Otol. Mag. 1878, plale ii. 



SECT, i § 3 ELEVATIOK-CSATER THEORY 243 

declivities till a cone may be so deeply trenched as to resemble a half- 
opened umbrella.^ 

The crater doubtless owes its generally circular form to the equal 
expansion in all directions of the explosive vapours from below. In 
some of the mud-cones already noticed, the crater is not more than a few 
inches in diameter and depth. From this minimum, every gradation of 
size may be met with, up to huge precipitous depressions, a mile or more 
in diameter, and several thousand feet in depth. In the crater of an 
active volcano, emitting lava and scorite, like Vesuvius, the walls are 
steep, rugged cliffs of scorched and blasted rock — red, yellow, and black. 
Where the material erupted is only loose dust and lapilli, the sides of the 
crater are slopes, somewhat steeper than those of the outside of the cone. 

The crater-bottom of an active volcano of the first class, when 
quiescent, forms a rough plain dotted over with hillocks or cones, from 
many of which steam and hot vapours are ever rising. At night, the 
glowing lava may be seen lying in these vents, or in fissures, at a depth 
of only a few feet from the surface. Occasional intermittent eruptions 
take place and miniature cones of slag and scoriie are thrown up. In 
some instances, as in the vast crater of Gurung Tengger, in Java, the 
crater'bottom stretches out into a wide level waste of volcanic sand, 
driven by the wind into dunes like those of the African deserts. 

A volcano commonly possesses one chief crater, often also many minor 
ones, of varying or of nearly equal size. The volcano of the Isle of 
Bourbon (or Reunion) has three craters.^ Kot infrequently craters api>ear 
successively, owing to the blocking up of the 
pipe below. Thus in the accompanying plan of 
the volcanic cone of the island of Volcanello 
(Fig. 55), one of the Lipari group, the volcanic 
funnel has shifted its position twice, ho that 
three craters have successively appeared ii|>om 
the cone, and partially overlap each other. It 
may be from this cause that some volcanic 
mountains are now destitute of craters, or in 
other cases, because the lava has welled up in 

dome form covered perhaps with masses of ,how1pH"three~iuc™i»7cnii»r'- 
acorite, but without the production of a definite 

crater. Mount Ararat, for example, is said to have no crater ; but so 
late as the year 1840 a fissure opened on its side whence a considerable 
eruption took place. The trachytic pnys of Auvergne are dome-shaped 
hilb without craters. 

Though the interior of modem volcanic cones can be at the best but 

• On the deandation of volcanic i-oue», «e H. J. Jolinaton-Lavi*, Q. J. Oeol. Sue. il. 

p. loa 

' For recent informalion regarding this volcanic island, tm R. vou Drasche, In Verhaadl. 
Oeol. JUichtantL 1875, ]>. 266, and in TBChemak'a Min. Mittheil. 1376 (3), p. 217 (4). 
p. 39, and hii work 'Die lusel Wunion (Bourbon),' 4to, Vienna, 1878. C. Velain. 
* Description giologique de Is Preaqn'ile d'Aden, de I'lla de U ftinnion, &c.,' Paris, 4to. 
1878 ; and his work, ' Le~ Volcmis,' ISS4. 




244 DYNAMICAL GEOLOGY book m pabii 

very partially examined, the study of the sites of long-extinct cones, laid 
bare after denudation, shows that subsidence of the ground has commoDly 
taken place at and round a vent. Evidence of subsidence has also been 
observed at some modern volcanoes (ante, p. 231). Theoretically two 
causes may be assigned for this structure. In the first place, the mere 
piling up of a huge mass of material round a given centre tends to prm 
down the rock underneath, as some railway embankments may be 
observed to have done. This pressiu*e must often amount to eeTflnl .^ 
hundred tons on the square foot. In the second place, the expolsioQ 
volcanic material to the surface may leave cavities underneath, into 
the overlying crust will naturally gravitate. These two causes 
as suggested by Mr. Mallet, afford a probable explanation of the 
sha])ed depressions in which many ancient and some modem venta 
to lie.i •"'•'^^ 

The following are the more important types of volcanic cones : ^ 




1. Cones of Non-Yolcanic Materials. — These are due to the discharge of 
other aeriform product through the solid crust without the emission of any troe 
lava. The materials ejected from the cavity are wholly, or almost wholly, partB of tke 
Kurrouuding rocks through which the volcanic pipe has heen drilled. Some of the eoifes 
surrounding the crater lakes {niaarc) of the Eifel consist chiefly of fragmentB of the 
underlying Devonian slates (pp. 200, 213). 

2. Toff-Cones, Cinder-Cones. — Successive eruptions of fine dust and stones^ often 
rendered i)a8ty by mixture vdth the water so copiously condensed during an eruption, 
form a cone in which the materials are solidified by pressure into tuff. Cones made np 
only of loose cinders, like Monte Nuovo in the Bay of Baiae, often arise on the flanks or 
round the roots of a great volcano, as happens to a small extent on Vesuvine, and on a 
larger scale ujwn Etna. They likewise occur by themselves apart from any lava-prodnoing 
volcano, though usually they atford indications that columns of lava have risen in their 
funnels, and even now and then that this lava has reached the surface. 

The cones of the Eifel district have long been celebrated for their wonderjful perfec- 
tion. Though small in size, they exhibit A^ith singular clearness many of the leading 
features of volcanic structure. Those of Auvergne are likewise exceedingly instmctive.' 
The high plateaux of Utah are dotted with hundreds of small volcanic cinder-cones, the 
singular [>o8itions of which, close to the edge of i)rofound river-gorges and on the upthrow 
side of faults, have already (p. 204) been noticed. Among the Carboniferous volcanic 



* Mallet, Q. J. Oed, Soc. xxxiii. p. 740. See also the account of * * Volcanic Necks,'* in 
Book IV. Part VII. 

2 Von Seebach {Z. JJeuisch. Geol. Ges. xviil 644) distinguished two volcanic types, let, 
Fkddfd Vofcam>€s (Strato-Vulkane), comix>sed of successive sheets of lava and tnfis, and 
embracing the great mjyority of volcanoes. 2nd, Dome Volcanoes^ forming hills com- 
posed of homogeneous protrusions of lava, with little or no accompanying fragmentary 
discharges, without craters or chimneys, or at least with only minor examples of these 
volcanic features. He believed that the same volcano might at different periods in its history 
belong to one or other of these types — the determining cause being the nature of the erupted 
lava, which, in the case of the dome volcanoes, is less fusible and more viscid than in that of 
the bedded volcanoes. (See below, under " Lava-cones.") 

^ For Auvergne, see works cited on p. 219. For the Eifel, consult Hibbert, 'History 
of the Extinct Volcanoes of the Basin of Neuwied on the Lower Rhine,' Edin. 1832. 
Von Dechen, ' Geoguostischer Fiihrer zu dem Laacher See,* Bonn, 1864. ' Geognoatitelier 
Flihrer in das Siebengebirge am Rhein,' Bonn, 1861. 



VOLCANIC CONES 



roclcB of oentral Scotland tlie atiimpa of ancient tiiff-coneB, frequenti; with a central core 
of liuatt, or with dykes sod veine of that rock, are of common occurrence.' 

The naterials of a toff-cone are arranged in more or leu regularly stratified lieds. 



-^:5SI**t 




nfrSrt.— ViewoftheTt 



On the out«r side, tliey dip down tlie slopes of the cone at the average angle of repose, 
which may range between 30° and 40°. From the summit of the crater-lip they likewise 
dip inward toward the erater-bottom at similar angles of inclination (Fig. 57). 

3. Hitd-Conu resemble tulT-cones in form, but are usually smaller iu size and less 
steep. They are produced hy the hardening of auccessire outpourings of mud from tlic 
oriGcee already described (p. 238). In the region of the Lower Indus, where they are 
abundantly distributed over an area of 1000 square miies, some of them attain a height 
of 100 feet, with cratera 30 yards across.' 




4. LftTA-^MinM. — Volcanic cones composed entirely of lava are comparatively rare, 
bnt occur in aome younger Tertiary and modem voloanoea. Fouijue descrilws tlie lava 
of 1806 at Santorin as having formed a dome-shaped elevation, Hewing out quietly and 

I Tram. Roy. Sk. Hdin. iiii. p. 455. Ste posUa. Book IV. Part VIl. 
' Lyell, ■ Principles,' ii. p. 77. 



246 DYNAMICAL aEOLOGY book hi pami 



ragiidty without fxpltHiaiiM. Af(«r iieveral days, hawever, its etiiistaon vai Mcompaokd 

with copious discharges of fraijmeutary malerialn and the formation of several uratOTidaa 

, tiioutha on the toji of the dome. Wliere lava ]>08aefiaea extreme liquidi^, 

I and given rine to little or no fragmciitarj' matter, it may build up a flat 

I'Oiie as ill the remarkable examples described by Dana from the Hamli 

iHlaiidnJ On the mmmlt of llauua Loa (Fig. 68), a flat Un-cone 13,7N 

feet above the aea, lies a crater, which in its deejiest part is about 8000 

feet broad, nith vertical walls of strati&ed lava riding on one nde to a 

height of 7S4 feet above the black lava-plain of the rrater-bottom. From 

the edges of this elevated cauldron, the mountain slopes outward at u 

angle of not more than fl', until, at a level of about 10,000 feet lower, it« 

auifscc is indented hj tlie vast pit-crater, Kilauea, abont two tnilea long^ 



^'S^> 




(Dana, 1S11.>) 



Biiil nearly a mile broad. So Ion- are the siiriiinudiiig slo]ies that thesr 
vast eiaters have been compared lo o]ien iiuarricii on a hill or moor. The 
biittom of Kiianea is a Iava-]>lain, clntted with lake^ of extremely Huii) 
lava in constant eliuUition. The level of the lava box varied, for the walli 
surrounding the tier)' Hood consist of l)nls of similar lava, and are marked 
by leilKcs or platforms (Fig. 5B) indicative of former sucooasive heighla of 
lava, as lake terraces show former levels of water. In the accompanying 
wrtion {Via. 80) the walls rising almve the Inivcr jut ipp') were found to 



Kiii. ciO,-Swti(inDfLav«-l«™cc«in Kil.u,^ (D«n»). 

be 'ii-i feet hiRh, those bounding the higher terrace (o n « o') were 960 
feet high, all being composed of innamcrable beds of lava, as in cliffs of 
stratified rocks. Much of the l«ttom of the lower lara-plain has been 
i:ni»teii over liy the solidification of the molten rock. But large areas, 
which shift their {losition froni time to time, remain in perpetual rapid 
ebullition. The glowing flood, as it boils up with a fluidity more like that 
of water than what is commonly shown by molten rock, surges against 



{■■pod of i: -^ Exploring Kritr-lilioa, 

See the works cited on p. 205. 
it niajis showing the variations of this a 



8-42, t 



id Dana's ' Character- 



i 'Choractattitic*.' 



VOLCANIC CONES 



247 



the suiToanding terrace walls. Large segments of the cliSa undermined by the fUaion of 
their b«se, fall at intervals into the fiery wavea and are soon inelled. Recent obeervatioDS 
by Captain DattoD point to a diminution of the actiritjof tliia lava-ctater. In Iceland, 
and in the Western Territories of North America, low domcB of laTa appear to mark the 
T«nta from which exteunve basalt-floods have issued. 

Where the lava assumes a more viscid character, as in trachyte and liparite, dome- 
shaped eminences may be protruded. Ah the moHs increases in size by the advent of 
Ireah material injected from below, the outer layer will be pushed outward, and sQcceuJve 
shells will in like manner be enlarged as tlie eruption advances. On the cessation of 
discharges, we may conceive that a volcuiic hill formed' in this way will present an 
onion-like arrangement of its component sheets of rock. Mors or less perfect examples 
of this structure have been observed In Bohemia, Auvergne, and the Eifel.' The 
trachytic domee of Aavergne form a conspicuous feature among the cinder cones of that 
region. Hnge conical protuberances of granophyre occur among the Tertiary volcanic 
rocks of the Inner Hebrides, and similar hills of liparite rise through the basalts of Iceland. 

6. CanM of Tuff and Lmva.— This is by far the most abundant type of volcanic 







Fig 



rian or til 









and includes the great volcanoes a! the globe Beginning perhaps as mere 
luff-cones, these eiTiinenoes have grailually been built np by successive outpourings of 
lava from dtfTerent side.'t, anil by showers of dust and scoriae. At first, the lava, if the 
sides of the cone are strong enough to resist its pressure, may rise until it overflows 
from the crater. Subsequently, as the funnel becomes choked up, and the cone is shat- 
tered by repeated explosions, the lava finds egress from different fissures and openings 
an the cone. As the mountain increases in height, the number of lava-currents from 
its summit will usually decivase. Indeed, the taller a volcanic cone grows, the less 
frequently as a rule does it enipt. The lofty volcanoes of the Andes have each seldom 

' E. Reyer {Jahiii. Oeol, Reichx. 1879. p. ^63) has experimentally imitated the process 
of extrusion by forcing np plaster of Paris through a hole in a board. For drawingH of the 
Pay de Sarcouy and other ilome-shajwd hills which presumably have had this mode of 
origin, see Scrope's ' Geology nuU Extinct Volcanoes of Central France.' Refer al»o to the 
lemarki already made nn the liqnidity of lava (nn(f, pp. 322-S), aud the accoont of 
" Valkanische Kuppen." poslta, p. 25S. 



S48 



DTKAMICAL GEOLOGY 



BOOK in PABTI 



1«eu more thau once in eruption duriug > century. The pe&k of Tenerifle (Fig. 61) m 
three times active daring S70 jesrs prior to 178S.' The earlier efforts of a Tolcano tnd 
to increase its height, ae well as its breadth ; the later eruptiona chieflj angmgnt ttt 
breadth, and are often apt to diminish the height by blowing away the nppetpartoftlit 
cone. The formation of liasurca and tlie consequent intmaion of a network of IsTk-dfkM, 
tend to bind the frameworlc of the volcano and strengthen it against subsequent ezplOMU. 
In thisway, a kind of oscillation is established in the form of the cone, periodaofaattr 
eruptions being succeeded by others when the emissions take place only latnsUy (Mir, 
p. 210). 

One consequence or lateral eruption is the formation of minor parasitic cones on the 




I. Uvi 



flanks of the parent volcano (p. 1S2). Those on Etna, more than 200 in number, an 
really miniature volcanoes, some of them reaching a height of 700 feet (Fig. 62), As the 
lateral vents successively become extinct, the cones are buried under sheets of lava and 
showers of debris thronii out from younger oi>enings or from the parent cone. It some- 
times happens tliat the original funnel is disused, and that the eruptions of the volcaito 



' For a recent ai 



it of Teoeriffe, lee A. Rothpleti, I'ettrmanWi iliUhtU. x\ 



'■ (18W, 



S3 



SUBMARINE VOLCANOES 



MM fivm a newer main vent, Veauriua, for exanipU (48 bIiowii 
m the uto of a portion of the rim of the more ancient 
«h Urgw vent of Monte Sommo. The present crater 
I liei to the north-vest of the former vaster crater. 
»t^ little example of this shifting furnished by Vol- 
haa been tlready noticed (p. 243]. 
ile, therefore, a volcano, and more particularly one of 
le, throwing out both lava and fragmentary materials, 
e to continual modification of its external form, as the 
e eruptJona, its contonr is likewise usually 
I alteration by the effects of ordinary 
bene erosion, aa well aa from the condensation of the 
: vapours. Heavy and sudden floods, produced by the 
aiufall consequent upon a copious discharge of steam, 
iwn the slopes with such volume and force as to cut 
lilies in the loose or only partially consolidated tuffs 
irice. Ordinary rain continues the erosion until the 
lopes, unless occasionally renewed by fresh showers of 
1, aasame a curiously furrowed aspect, like a half-ojiencd 
A, the ridges being separated by furrows that narrow 
B towards the summit of the cone. The outer declivi- 
Uonte Somma afford an eicellent ilhiatrstion of this 
surface, the numerous raviuea on that side of the 
in presenting instructive sections of the pre-hiatoric 
id tu^ of the earlier and more important iteriod in the 
of this volcano. ' Similar trenches have been eroded 
Muthem or Vesuvian side of the original cone, bat 
ave in great measure been filled up by the lavas of the 
: mountain. The ravines, in fact, form natural chan- 
■ the lava, as may unfortunately be seen round the 
0) observatory. This building is ]>laced on one of the 
between two deep ravines ; but the lava-streams of 
'ewa have poured into these ravines on either side, and \ 

dly filling them up. 

bmarltie Volcanoes. — It is not only on the 
: of the land that volcanic action ehows itself, 
es place likewise under the sea, and as the 
ical recprds of the earth's \>a»t history arc 

marine fonnations, the characteristics of sub- 
) volcanic action have no small interest for the 
ist. In a few instances, the actual outbreak 
ibmarine erujition has been witnessed. Thus, 
i early summer of 1783, a volcanic eruption 
lace about thirty miles from Cape Keykjaiiaes 
! west coast of Iceland. An island was built 
im which fire and smoke continued to issue, 

less than a year the waves had washed the 
Dumice away, leaving a submerged reef from 
) thirty fathoms below sea-level. About a 

' See H. J, Johnston- Lav i«, Q. J. Gtol. Sm. il. p. 103. 



y 



il 

■If 
hi 

jsl 

I 2-a 

an 



Hit 

s £,^ 

E I ■< . 

ill 
5|I 

s-sa 
I If- 



280 DYNAMICAL GEOLOGY bookotpaoi 

month after this eruption, the frightful outbreak of Sk&ptar JokaD, 
already refened to (p. 233), began, the diBtance of this mountain from 
the submarine vent being nearly 200 mileB.' A century afterwards, m 
in July 11384, another volcanic island is said to have been thrown iq) 
near the same spot, having at first the form of a flattened cone, but soon 
yielding to the power of the bi'eakcrs. Many submarine eruptions have 
taken place within historic times in the Mediterranean. The niost 
noted of these occurred in the year 1831, when a new volcanic idand 
(Graham's Island, lie Julia) was thrown up, with abundant dlschujB 
of steam and showei-R of scoriee, between Sicily and the coast of Afaio. 
It reached an extreme height of 200 feet or more above the sea-lerel 
(800 feet above sea-bottom) with a circumference of 3 miles, but on 




'Sktlcliof submuiiie volunlc cruptLun(ttabriiu l>lind)a)rst Hichul'i, Jnne 1S1L 



the cessation of the eruptions, was attacked by the waves and soon 
demolished, leaving only a shoal to mark its site.- In the year 1811, 
another island was formed by submarine eruption of the coast off 
St. Michaela in the Azores (Fig. 04). Consisting, like the Mediterranean 
example, of loose cinders, it rose to a height of about 300 feet, with 
a circumference of about a mile, biit subsequently disappeared.' In 
the year 1796 the island of Johaima Bogoslawa, in Alaska, appeared 
above the water, and in four years had grown into a large volcanic 
cone, the summit of which was .^000 feet above sea-level.* 

' Ljell, ' Priiiciiiks,' ii. p. 49. 

' Pkil. Tram. IS32. CoDBtunt Piijvojit. Ann. ilia Set. Xal. ixir. JTAn. Sac OM. 
France, ii. ji. 91. JIiTcalli's ' Vulcani, fcc.,' p. 117. For a recent labmirine arnptkni in 
Ihe Mediterranean, we Kioco, Coiiiiif. rriiil. Xov. 23ril, 1891, 

' De In Beclie, ' Geological Oliserter,' p, "0, * D. Forbes, G*ot. Mag, vU, p. 923. 



i«3 



SUBMARINE VOLCANOES 



[JufortunAtely, the phenomena of 
it volcanic eruptions under the 
ire for the most part inacceBeibltj. 
1 and there, as in the Bay of 
les, at Etna, among the islands of 




ca, cr Buntorin ; h, Th^nulii ; 
toKmlmcni. Tlie llgiirHi Ui 
>mi,thf I 

xreek Archipelago, and at Tahiti, 
ktion of the sea-Wl has taltcn 
;, and brought Xo the siirface beds 
iff or of lava which have consoli- 
1 under water. Both Vesuvius 
Etna began their career as sub- 
ne volcanoes.' It will be seen 
the accompanying chart (Fig. 
that the Islands of Santorin and 
aaia form the unsubmergcd por- 
of a great crater-rim rising round 
ater which descends 1278 feet 
w sea- level. The matci'ials of 
) islands consist of a nucleus 
narhles and schists, nearly 
>d under a pile of tufTs (trass), a 
m, and sheets of lava, the 
ed character of which is well 
n in the accompanying sketch 
aee, M regan 
327. 




252 DYNAAflCAL GEOLOGY book iif paw i 

by Admiral Spratt (Fig. 66), who, with the late Professor Edward Forbes, 
examined the geology of this interesting district in 1841. They found 
some of the tuffs to contain marine shells, and thus to bear witness to an 
elevation of the sea-floor since volcanic action began. More recently the 
islands have been carefully studied by various observers. K. von Fritsch 
has found recent marine shells in many places up to heights of nearly 
600 feet above the sax. The strata containing these remains he 
estimates to be at least 100 to 120 metres thick, and he remarks 
that in every case ho found them to consist essentially of volcanic debris 
and to rest upon volcanic rocks. It is eWdent, therefore, that these shell- 
Ijearing tuff's were originally deposited on the sea-floor after volcanic 
action had begun here, and that during later times they were upraised, 
together with the submarine lavas associated with them.^ Fouqu^ con- 
cludes that the volcano formed at one time a large island with wooded 
slopes and a somewhat civilised human population, cultivating a 
fertile valley in the south-western district, and that in prehistoric times 
the tremendous explosion occurred whereby the centre of the island was 
blown out. 

The similaritv of the structure of Santorin to that of Somma and 
Etna is obvious. Volcanic action still continues there, though on a 
diminished scale. In 1866-67 an eruption took place on Neo E^imeni, 
one of the later-formed islets in the centre of the old crater, and greatly 
added to its area and height. The recent eniptions of Santorin, which have 
])een studied in great detail, are specially interesting from the additional 
infoimation they have supplied as to the nature of volcanic vapours and 
gases. Among these, as already stated (p. 196), free hydrogen plays an 
important part, constituting, at the focus of discharge, thirty per cent of 
the whole. By their ei-uption under water, the mingling of these gases 
with atmospheric air and the combustion of the inflammable compounds 
is there prevented, so that the gaseous discharges can be collected and 
analysed. Probably were operations of this kind more practicable at 
terrestrial volcanoes, free hydrogen and its compounds would be more 
abundantly detected than has hitherto been possible. 

The numerous volcanoes which dot the Pacific Ocean, probably in 
most cases began their career as submarine vents, their eventual appear- 
ance as subaerial cones being mainly due to the accumulation of erupted 
material, but also jiartially, as in the case of Santorin, to actual upheaval 
of the sea-bottom. The lonely island of St. Paul (Figs. 67 and 69), 
lying in the Indian Ocean more than 2000 miles from the nearest land, 
is a notable example of the summit of a volcanic mountain rising 
to the sea-level in mid -ocean. Its circular crater, broken down on 

1 See Fritsch, Z. Deuisch. Gfol. Ges. xxiii. (1871), pp. 125-213. The moat complete 
and elaborate work is Fouque's monograph (already cited), ' Santorin et ses Smptiona,' 
Paris. 4to, 1880, where copious analyses of rocks, minerals, and gaseous emanatioiis. 
with maps and numerous admirable views and sections, are given. In this Tolame a 
bibliography of the locality will be found. Compare C. Doelter on the Ponza Islands. 
IknlcAch. Ahul. Wisgrnsdi. Vienna, xxxvi. p. HI. Sitz. Akad. iriMfftMA. Yienot. 
Ixxi. (1875), p. 49. 




tigs SUBMARIXE VOLCANOES 853 

Qortb-eaat side, is filled with water, having a depth of 30 

Observations by R. von Drasche 
^e shown that at Bourbon {Reunion), 
ring the early submarine eruptions of 

t volcano, coarsely crystalline rocks f:\''\''i^^t^^2k* ")" 

bbro) were emitted, that these were '' 

«eeded by andoaitic and trachytic lavas : 
*. that when the vent rose above the 
, basalts were poured out.* Fouque 
<erves that at Santorin some of the 
•\j submarine lavas are identical with 
ne of later subaerial origin, but that 
I greater part of them belong to an 
irely different series, being acid rocks, fi«. bt-— VuiouiccimterofBi, p»uii«i«nj, 
onging to the group of hornblende- indisn ocan. 

lesites, while the subaerial rocks are angite-andesites. The acidity of 
jse lavaa has been largely increased by the infusion into them of much 
ca, chiefly in the form of opal They differ much in aspect, being 
aetimes compact, scoriaceous, hard, like millstone, with perlitic and 
lerulitic structures, while they frequently present the characters of 
B8 impregnated with opal and zeolites. Among the fragmental 
ctions there occur blocks of schist and granitoid rocks, probably 
vesenting the materials below the sea-floor through which the first 
iloeion took place (pp. 200, 213, 244). During the eruption of 1866 
ne islets of lava rose above the sea in the middle of the bay, near the 
ivo vent. The rock in these cases was compact, vitreous, and much 
eked,* 

Among submarine volcanic formations, the tulTs differ from those laid 
(TO on land chiefly in their organic contents ; but partly also in their 
re distinct and originally less inclined bedding, and in their tendency 
the admixture of non-volcanic or ordinary mechanical sediment with 
! volcanic dust and stones. No appreciable difference either in 
:emal aspect or in internal structure seems yet to have been established 
;ween subaerial and submarine lavas. Some undoubtedly submarine 
-aa are highly scoriaceous. There is no reason, indeed, why slaggy 
•a. and loose, non-buoyant scoriie should not accumulate under the 

' For a g«nenl accoiiut of the volcanic isUmls of the ocean, see Darwin's ' Vokanic 
iBdi,' Snd edit. 1S7S. For the Philippine volcanoea, see R. von Draiche, TsrAemali'» 
utmlogiiiAe MiWieii. 1 876 ; Semper's ' Die Philippinen und ihre Bevrohner, ' WUrz- 
g, 1869. For the Kurile Islands, J. Milne, Geol. Mag. 1879, 1860, 18S1 ; Volcanoea 
J«y of BeDgal (Barren Islan.l. !;c.), V. Ball, Qtol. Mag. 1879, p. 18 ; 1888, p. ^04 ; F. 
SI«llet, Man. Orel. Sun: India, iii. part iv. St. Paul (Indian Ocean), C. va»ln, Aaoc. 
in. 1875, p. 581 ; ' Mission i. Tile St. Paul," 1879 ; 'Description geologiqne de la 
■qa'ilfl d'Aden,' &c.. 4to, Paris, 1S7S ; and ' Les Volcans,' 1684. For Iile of Bourbon, 
■athorities citeil on p. 243, and for Hawaii, the references on p. 205. 

* Tschermok's MinenilogUche MUtkeil. 1876, pp. 42, 1S7. A similiir stmetnre occuis 
lUmA (Coheo, .Vewj JiArb. 1879, p. 482) and in SL Paul (Velain as above cited). 

* Fonqne, ' Sontorid. ' 



254 IiYKAMlCAL GEOLOGY BOOsmPiBii 

pressure of a deep coliunn of the ocean. At the Hawaii IslandB, on 25di 
February 1877, masses of pumice, during a submaiine volcanic explodon, 
were ejected to the surface, one of which struck the bottom of a boat 
with considerable violence and then floated. When we reflect, indeed, 
to what a considerable extent the bottom of the great ocean-buini ii 
(lotted over with volcanic cones, rising often solitary from profoond 
depths, we can believe that a lai-ge proiwiiion of the actual enipdon 
in oceanic areas may take place under the sea. The immense abunduce 
and wide diffusion of volcanic detritus (including blocks of pumice) over 




the bottom of the Pacific and Atkntic oceans, even at distances remote 
from land, as made known by the voyage of the CkalUoffer^ doubt 
less indicate the prevalence and pei-sistenco of submarine volcamc 
action, even though, at the same time, an extensive difluaion of volcanic 
debris from the islands is admitted to Iw effected liy winds and ocewi- 
currents. 

Volcanic- islands, unless continually augmented by renewed eruptions. 




iiK-pIn Rock. * itwli of Ivrdrr nick left by thf bf> : 
tththiGlicrriuKc) liy w»vf.. aud »ul«rtal wtnU-.j 



:iM (Ca]i1. ItlukiTDal Id Admlnl^ Chut). 
liguon (HC Pig. «n; 



are attacked by the waves and cut down. Graham's Island and the 
other examples above cited show how rapid this disappearance may 



BBCT. i § 3 FISSURE^ERUPTIONS 265 

be. The island of Volcano has the base of its slopes truncated by a line 
of cliff due to marine erosion. The island of Teneriffe shows, in the 
same way, that the sea is cutting back the land towards the great cone 
(Pig. 68). The island of St. Paid (Figs. 67, 69) brings before us in a 
more impressive way the tendency of volcanic islands to be destroyed 
unless replenished by continual additions to their surface. At St. 
Helena lofty cliffs of volcanic rocks 1000 to 2000 feet high bear witness 
to the enormous denudation whereby masses of basalt two or three miles 
long, one or two miles broad, and 1000 to 2000 feet thick, have been 
entirely removed.^ 

ii. Fissure (Massive) Eruptions. 

Under the head of massive or homogeneous volcanoes some geologists 
have included a great number of bosses or dome-like projections of once- 
melted rock which, in regions of extinct volcanoes, rise conspicuously 
above the surface without any visible trace of cones or craters of 
fragmentary material They are usually regarded as protrusions of 
lava, which, like the Puy de Ddme in Auvergne, assumed a dome-form 
at tiie surface without spreading out in sheets over the surrounding 
coantry, and with no accompanying fragmentary discharges. But the 
mere absence of ashes and scoriae is no proof that these did not once 
exist, or that the present knob or boss of lava may not originally have 
solidified within a cone of tuff which has been subsequently removed in 
denudation. The extent to which the surface of the ground has been 
changed by ordinary atmospheric waste, and the comparative ease with 
which loose volcanic dust and cinders might have l:>een entirely removed, 
require to be considered. Hence, though the ordinary explanation is no 
doubt in some cases correct, it may be doubted whether a large propor- 
tion of the examples cited from the Rhine, Bohemia, Hungar}^ and 
other regions, ought not rather to be regarded like the " necks " so 
abundant in the ancient volcanic districts of Britain (Book IV. Part VII.) 
as the remaining roots of ordinary volcanic cones. If the tuff of a 
cone, up the funnel of which lava rose and solidified, were swept away, 
we should find a central lava plug or core resembling the volcanic 
"heads" (vidkanisclie Knppen) of Germany. Unquestionably, lava has 
in innumerable instances risen in this way within cones of tuff or 
cinders, partially filling them without flo^ring out into the surrounding 
country.2 

But while, on either explanation of their origin, these volcanic " heads " 
find their analogues in the emissions of lava in modern volcanoes, there 
are numerous cases in old volcanic areas where the eruptions, so far as 
can now be judged, were not attended with the production of any central 
cone or crater. Such emissions of lava may have resembled those which 

^ Darwin, * Volcanic Islands,' p. 104. For a more detailed account of this island, see 
J. C. Melli.Hs' 'St. Helena,' London, 1875. 

* Von Seebach, Z. Deutsch. OeoL Ges. xviii. p. 643. F. von Hochstetter, Neues 
JaKrh. 1871, p. 469. Reyer, Jahrb. K, K, OeoL ReichsansUdU 1878, p. 81 ; 1879, p. 
463. 



256 TiYSAMICAL GEOUJGY book ni pabt I 



in recent times have occurred at the Hawaiian volcanoes, where enormoiu 
accumulations of lava have gradually been built up into flat domefl, of 
which Mauna Loa rises to a height of 1 3,675 feet. Vast floods of remark- 
ably liquid basic lava have from time to time flowed out tranquilly without 
explosion or eaithquake, and with no accom|)animcnt of fragmental dis- 
charges. These currents of molten rock have spread out into wide sheets, 
sloping at so low an angle that they look horizontal. The lower and older 
portions of them have l>een eroded by streams so as to present escarp- 
ments and outliers not unlike those of western North America or the 
older basaltic plateaux of Britain and India.^ 

The most stupendous modern basaltic-floods of Iceland issued from 
vents along a fissure. Acconling to Thoroddsen the post-glacial lavsr 
fields of Odadahraun, covering an area of about 4390 square kilometres, 
have issued from about 20 distinct vents, while in the east of Iceland the 
lava has flowed from the lips of fissures.- It would seem that for the 
discbarge of such wide and flat sheets of lava, great mobility and 
tolerably complete fusion of the molten mass is necessary. The 
phenomenon occurs among the more basic lavas (basalts, &c) rather than 
among the more lithoid acid lavas (trachytes, rhyolites. Sec.) 

In former geological ages, extensive eruptions of lava, without the 
accompaniment of scoriae, with hardly any fragmentary materials, and 
with, at the most, only flat dome-shaped cones at the points of emission, 
have taken place over wide areas from scattered vents, along lines or 
systems of fissures. Vast sheets of lava have in this manner been poured 
out to a depth of many hundred feet, completely burying the previous 
sui*face of the land and forming wide plains or plateaux. These truly 
" massive eruptions " have been held by Kichthofen ^ and others to 
represent the grand fundamental character of volcanism, ordinary volcanic 
cones being regarded merely as i)arasitic excrescences on the subterranean 
lava-reservoirs, \qy\ much in the relation of minor cinder cones to their 
parent volcano.* 

Though a description of these old fissure or massive eruptions ought 
pn>|x»rly to Ihj included in Book IV., the subject is so closely connected 
with the dynamics of existing active volcanoes that an account of the 
subject may be given here. Perhai)s the most stupendous example of this 
type of volcanic structiue occims in Western North America. The extent 
of country which has been flooded with hisalt in Oregon, Washington, 
California, Idaho, and Montana has not yet been acciurately surveyed, but 
has been estimated to cover a larger area than France and Great Britain 
combined, with a thickness averaging 2000 but reaching in some places 
to 3700 feet.^ The Snake River plain in Idaho (Fig. 70) forms part of 

^ For a gi-aj>hic account of the Hawaiian lava-fieUls, see Captain Dutton, Foarth Animal 
Report, U. S. Geol. Survey for 1882-83. See also Dana's * Characteristics of Volcanoes.' 

» See W. L. Watts' '' Across the Vatna Jokull," Proc. Roy. Qtiig. Soc 1876. W. G. 
Lock, Oeol. Mag. 1881, p. 212 ; and papers by Thoro<ldsen and Helland, quoted antej pw 202. 

' Trans. Akatl. Sci. California, 1868. 

"* Proc. Ruy. Phys. Sac. JS<lin. v. 236 ; Sature, xxiii. p. 3. 

* J. LeConte. Amer. Journ. Sci. 3rd ser. vii. (1874), 167, 259. 



nor. i § 3 



FISSURE-ER UPTIONS 



257 



this lava-flood, Surrounded on the north and east by lofty mountains, it 
BtretcheB westward as an apparently boundless deeort of sand and bare 
sheets of black basalt. A few streams descending into the plain from the 
hills are soon swallowed up and tost. The Snake River, however, flows 
across it, and has cut out of its lava-beds a series of picturesque gorges 
and rapids. Looked at from any point on it« surface, it appears as a vast 
level plain like that of a lake-bottom, though more detailed examination 
may detect a slope in one or more directions, and may thereby obtain 
evidence as to the sites of the chief openings from which the basalt was 
poured forth. The uniformity of suriace has been produced either by the 
lava flowing over a plain or lake-bottom, or by the complete effacomont of 
an original and undulating contour of the ground under hundreds of feet 
of volcanic rock in successive sheets. The lava rolling up to the base of 




the mountains has followed the sinuosities of their margin, as the waters 
of a lake follow its promontories and bays. The author crossed the 
Snake River plain in 1879, and likewise rode for many miles along its 
northern edge. He found the surface to be everywhere marked with 
low hummocks or ridges of bare black liasalt, the surfaces of which 
exhibited a reticulated pavement of the ends of columns. In some places, 
there was a perceptible tendency in these ridges to range themselves in 
one general north-easterly direction, when they might be likened to a 
series of long, low waves, or ground-swells. In many instances the crest 
of each ridge had cracked open into a fissure which presented along its 
walls a series of tolerably symmetrical columns (Fig. 70). That these 
ridges were original undulations of the lava, and had not been produced 
by erosion, was indicated by the fact that the columns were perpendicular 
to their surface, and changed iu direction according to the form of the 
ground which was the original cooling surface of the lava. Though the 



258 nVXAMIGAL GEOLOGY book hi part x 



basalt was sometimes vesicular, no layers of slag or scorisB were anywhere 
observed, nor did the surfaces of the ridges exhibit any specially scorifomi 
character. 

There are no great cones whence this enormous flood of basalt could 
have flowed. It probably escaped from orifices or fissui^es still concealed 
under the sheets which issued from them, the points of escape being 
marked only by such low domes as could readily be buried under the 
succeeding eruptions from other vents.^ That it was not the result of 
one sudden outpouring of rock is shown by the distinct bedding of the 
basalt, which is well marked along the river ravines. It arose from what 
may have been, on the whole, a continuous though locally intermittent 
welling-out of lava, probably from vents on many fissures extending over 
a wide tract of Western America duiing a late Tertiary period, if, indeed, 
the eruptions did not partly come within the time of the human occupation 
of the continent. The discharge of lava continued until the previous topo- 
graphy was buried under some 2000 feet of lava, only the higher summits 
still projecting above the volcanic flood.- At a few points on the plain 
and on its northern margin, the author observed some small cinder cones 
(Fig. 70). These were evidently formed during the closing stages of 
volcanic action, and may be compared to the minor cones on a modem 
volcano, or better, to those on the surface of a recent lava-stream. 

In Europe, during older Tertiary time, similar enormous outpourings 
of basalt covered many hundreds of square miles. The most important of 
these is that which occupies a large part of the north-east of Ireland, 
and in disconnected areas extends through the Inner Hebrides and the 
Faroe Islands into Iceland. Throughout that region, the paucity of 
evidence of volcanic vents is truly remarkable. So extensive has been 
the denudation, that the inner structure of the volcanic plateaux has been 
admirably revealed. The ground beneath and around the basalt-sheets 
has been rent into innumerable fissures which have been filled by the 
rise of basalt into them. A vast number of basalt-dykes ranges from 
the volcanic area eastwards across Scotland and the north of England and 
the north of Ireland. Towards the west the molten rock reached the 
surface and was poured out there, while to the eastward it does not 
appear to have overflowed, or, at least, all evidence of the outflow has 
been removed in denudation. When we reflect that this system of dykes 
can be traced from the Orkney Islands southwards into Yorkshire and 
across Bntain from sea to sea, over a total area of probably not less than 
100,000 square miles, we can in some measure appreciate the volume 
of molten basalt which in older Tertiary times underlay large tracts of 
the site of the British Islands, rose up in so many thousands of fissures, 
and poui'ed forth at the surface over so wide an area in the north-west.' 

In Africa, basaltic plateaux cover large tracts of Abyssinia, where by 

^ Captain Dutton has remarked the absence of any conspicuous feature at the •onroes 
from which some of the largest lava-streams of Hawaii have issued. 

'■^ Professor J. LeConte believes that the chief fis.sures opened in the Cascade and Blue 
Mouutaiji Ranges. Amer. Jnnm. Sci. 3rd series, \'ii. (1874), p. 168. 

' Trans, Roy. Soc. Edin. xxxv. (1888), p. 21. 



SECT, i S 4 DISTRIBUTION OF VOLCANOES 259 

the denuding effect of heavy rains they have been carved into picturesque 
hills, valleys, and ravines.^ In India, an area of at least 200,000 square 
miles is covered by the singidarly horizontal volcanic plateaux of the 
" Deccan Traps " (lavas and tuffs), which belong to the Cretaceous period 
and attain a thickness of 6000 feet or more.^ The underlying platform 
of older rock, where it emerges from beneath the edges of the basalt table- 
land, is found to be in many places traversed by dykes ; but no cones 
and craters are anywhere visible. In these, and probably in many other 
examples still undescribed, the formation of great plains or plateaux of 
level sheets of lava is to be explained by " fissure-eruptions " rather than 
by the operations of volcanoes of the familiar " cone and crater " type. 

§ 4. Geographical and geological distribution of 

volcanoes. 

Adequately to trace the distribution of volcanic action over the 
globe, account ought to be taken of dormant and extinct volcanoes, like- 
wise of the proofs of volcanic outbreaks during earlier geological periods. 
When this is done, we learn, on the one hand, that innumerable districts 
have been the scene of prolonged volcanic activity, where there is 
now no imderground commotion, and on the other, that volcanic out- 
bursts have been apt to take place again and again after wide intervals 
on the same ground, some modern active volcanoes being thus the 
descendants and representatives of older ones. Some of the facts 
regarding former volcanic action have been already stated. Others will 
be given in Book W. Part VII. 

Confining attention to vents now active, of which the total number 
may be about 300,^ the chief facts regarding their distribution over the 
globe may be thus summarised. (1) Volcanoes occur along the margins 
of the ocean-basins, particularly along lines of dominant mountain- 
ranges, which either form part of the mainland of the continents or extend 
as adjacent lines of islands. The vast hollow of the Pacific is girdled 
with a wide ring of volcanic foci. (2) Volcanoes rise, as a striking 
feature, from the submarine ridges that traverse the ocean basins. All 
the oceanic islands are either volcanic or formed of coral, and the 
scattered coral-islands have in all likelihood been built upon the tops of 
submarine volcanic cones. (3) Volcanoes are situated not far from the sea. 
The only exceptions to this rule are certain vents in Mantchouria and in 
the tract lying between Thibet and Siberia ; but of the actual nature of 
these vents very little is yet knoAvn. (4) The dominant arrange- 
ment of volcanoes is in series along subterranean lines of weakness, 

» Blanford's 'Abyssinia,' 1870, p. 181. 

' Medlicott and Blanford, * Geology of India,' p. 299. 

' Thia number is probably below the truth. Prof. J. Milne has enumerated in Japan 
alone no fewer than fifty-three volcanoes which are either active or have been active within 
a recent period. He remarks that, "if we were in a position to indicate the volcanoes 
which had been in eruption during the last 4000 years, the probability is that they would 
number seyeral thousands rather than four or five hundred." ' Earthquakes and other Earth - 
movements,' 1886, p. 227. Compare Fisher, ' Physics of Earth's Crust,' 2nd ed. chap. xxiv. 



260 DYNAMICAL GEOLOGY book m part i 

as in the chain of the Andes, the Aleutian Islands, and the Malay 
Archipelago. A remarkable zone of volcanic vents girdles the globe 
from Central America eastward by the Azores and Canary Islands to 
the Mediterranean, thence to the Red Sea, and through the chains of 
islands from the south of Asia to New Zealand and the heart of the 
Pacific. (5) On a smaller scale the linear arrangement gives place to one 
in groups, as in Italy, Iceland, and the volcanic islands of the great oceans. 

In the European area there are six active volcanoes — Vesuvius, Etna, 
Stromboli, Volcano, Santorin, and Nisyros. Asia contains twenty-four, 
Africii ten. North America twenty,^ Central America twenty-five, and 
South America thirty-seven.*' By much the largest number, however, 
occur on islands in the ocean. In the Arctic Ocean rises the solitary 
Jan Mayen. On the ridge se^mrating the Arctic and Atlantic basins, the 
group of Icelandic volcanoes is found. Along the great central ridge of 
the Atlantic bottom, numerous volcanic vents have risen above the sur- 
face of the sea — the Azores, Canary Islands, and the extinct degraded 
volcanoes of St. Helena, Ascension and Tristan d'Acunha. On the 
eastern border lie the volcanic vents of the islands off the African coast, 
and to the west those of the West India Islands. Still more remarkable 
is the development of volcanic energy in the Pacific area. From the 
Aleutian Islands southwards, a long line of volcanoes, numbering 
upwards of a hundred active vents, extends through Kamtschatka and 
the Kurile Islands to Japan,^ whence another numerous series carries 
the volcanic band far south towards the Malay Archipelago, which must 
be regarded as the chief centre of the present volcanic activity of our 
planet. In Sumatra, Java, and adjoining islands, no fewer than fifty 
active vents occur. The chain is continued through New Guinea and the 
groups of islands to New Zealand.* Even in the Antarctic regions, 
Mounts Ei'ebus and Ten*or are cited as active vents ; while in the centre 
of the Pacific Ocean rise the gi-eat lava cones of the Sandwich Islands. 
In the Indian Ocean, the Red Sea, and ofl* the east coast of Africa a 
few scattered vents appear. 

Besides the existence of extinct volcanoes which have obviously 
been active in comparatively recent times, the geologist can adduce 

^ For au account of tlie remarkable extinct volcanoes of Northern California, Oregon, and 
Washington Territory, see A. Hague and J. P. hidings. Ainer. Journ. Sci. xxvi. (1883), 
j>. 222. On Volcanoes of Mexico see H. Lenk, * Beitriige zur Geologic und Pabeontologie 
(ler Repbulik Mexico,' Leipzig, 1890 : of Central America, A. Dolfuiis and E. de Monsemt, 
'Voyage Gtologique. ' Paris, folio, 1868 ; K. von Seebach, Ahh. Kiin. Oes. Wiss, GUttiHgen^ 
xxxviii. (1892). 

•^ For a recent account of the volcanoes of the Andes of the Equator see Whympers 
'Travels Amongst the Great Andes.' 

•* For the volcanoes of Jajjau, besides papers (juote*! on p. 213, see W. J. Holland, 
Appalachid, vi. (1890), 109. E. Xanmann, Zeiisch, Deutsch. Geol. Gen. 1877, p. 864. Mr. 
Milne enumerates 100 active vents from the Kuriles to Kinshu (2000 miles). 

* The great eruption of Tarawera, New Zealand, in 1886, is describcil by Prof. A. P. W. 
Thomas, * Report on the Eruption of Tarawera,' published by the Government in 1888 : also 
Prof, llutton's * Rei>ort on the Tarawera Volcanic District, Wellington, 1887,' Qu€arL Jaum. 
Geol. Soc. xliii. (1887), p. 178. 



8ECT.i§4 DISTRIBUTION OF VOLCANOES 261 

proofs of the former presence of active volcanoes in many countries 
where cones, craters, and all the ordinary aspects of volcanic mountains, 
have long disappeared, but where sheets of lava, beds of tuff, dykes, and 
necks representing the sites of volcanic vents have been recognised 
abundantly (Book IV. Part VII.) These manifestations of volcanic 
action, moreover, have as wide a range in geological time as they have in 
geographical area. Every great geological period, back into pre-Cambrian 
time, seems to have had its volcanoes. In Britain, for instance, there 
were probably active volcanic vents in pre-Cambrian ages. The Archaean 
gneiss of N.-W. Scotland includes a remarkable series of dykes presenting 
some points of resemblance to the great Tertiary system. The Torridon 
sandstone of the same region, which is now known to be pre-Cambrian, 
contains pebbles of various finely vesicular porphyrites, and in one place 
includes a band of true tuff. In the lower Cambrian period the 
tuffs and diabases of Pembrokeshire were erupted. Still more vigorous 
were the volcanoes in the Lower Silurian period, when the lavas and tuffs 
of Snowdon, Aran Mowddwy, and Cader Idris were ejected. During 
the deposition of the Upper Silurian rocks a few volcanoes were active in 
the west of Ireland. The Lower Old Red Sandstone epoch was one of 
prolonged activity in central Scotland. The earlier half of the Carbon- 
iferous period likewise witnessed two distinct epochs of volcanic activity 
over the same region. In the earlier of those, lavas (andesites and 
trachytes) were poured out in wide level plateaux from many vents, 
while in the later, groups of minor cones like the puys of Auvergne were 
dispersed among the lagoons. During Permian time, more than a hundred 
small vents rose in scattered groups across the centre and south-west of 
Scotland, while a few similar points of eruption appeared in the south- 
west of England. No trace of any British Mesozoic volcanoes has been 
met with. The vast interval between Permian and older Tertiary time 
appears to have been a period of total quiescence of volcanic activity. 
The older Tertiary ages were distinguished by the outpouring of the 
enormous basaltic plateaux of Antrim and the Inner Hebrides.^ 

In France and Germany, likewise, Palaeozoic time was marked by the 
eruption of many diabase, porphyrite, and quartz-porphyry lavas. In 
Brittany, for example, Dr. Barrois has found a remarkable series of 
older Palaeozoic diabases and porphyrites with tuffs and agglomerates. 
He distinguishes four principal periods of eruption — 1. Cambrian and 
Lower Silurian ; 2. Middle and Upper Silurian ; 3. Upper Devonian ; 
4. Carboniferous.* The Permian period was marked in Germany and 
also in the south of France by the discharge of great masses of various 
quartz-porphyries. The Triassic period likewise witnessed numerous 
eruptions. But from that period onward the same remarkable quiescence 
appears to have reigned all over Europe, which characterised the 
geological history of Britain during Mesozoic time.^ In Tertiary time a 

* For a detailed summary of the volcanic history of Britain, see Presidential addreases 
to the Geological Society, Quart. Journ. Geal. Soc. xlvii. xlviii. (1891-92). 

* CarU Giol, dStaill. France, No. 7, 1889. 

' Some trifling exceptions to this general statement are said to occur. C. E. M. Rohrbach 



262 DYXAMICAL GEOLOGY book in part i 

prodigious outpouring of lavas, both acid and basic, continued from the 
Miocene epoch down even perhaps to the historic period. Examples of 
this great series are met with in Central France, the Eifel, Italy,^ Bohemia, 
and Hungary, almost to the existing period.^ Recent research has 
brought to light evidence of a long succession of Tertiary and post- 
Tertiary volcanic outbursts in Western America (Nevada, Oregon, Idaho, 
Utah, &c.) Contemporaneous volcanic rocks are associated with Palaeo- 
zoic, Secondary, and Tertiary formations in New Zealand, and volcanic 
action there is not yet extinct. 

Thus it can be shown that, within the same comparatively limited 
geogi*aphical space, volcanic action has been rife at intervals during a 
long succession of geological ages. Even round the sites of still active 
vents, traces of far older eruptions may be detected, as in the case of the 
existing active volcanoes of Iceland, which rise from amid Tertiary lavas 
and tuffs. Volcanic action, which now manifests itself so conspicuously 
along certain lines, seems to have continued in that linear development 
for protracted periods of time. The actual vents have changed, dying 
in one place and breaking out in another, yet keeping on the whole 
along the same tracts. Taking all the manifestations of volcanic action 
together, both modern and ancient, we see that the subterranean forces 
have operated along great lines in the earth's crust, and that the existing 
volcanoes form but a small proportion of the total number of once active 
vent*?. 

Looking broadly at the geological history of volcanic action we 
observe that, while there is eWdence of the protinision of both acid and 
basic materials from the remotest periods, the earlier discharges were 
preponderantly acid. In Britain, for example, the vast piles of lavas 
ejected during the Silurian period were mainly of a felsitic character, 
though considerable accumulations of andesites were not wanting. On 
the other hand, the wide sheets of lava poured out in this country 
during Tertiary time were chiefly basalts, the acid protrusions occurring 
for the most ]>art as dykes and bosses. A similar broad sequence has 
been observed in other countries. 

When, however, we proceed to consider more closely the nature of 
the successive eruptions during the continuance of one of the volcanic 
periods of which records are preserved among the geological formations^ 
we discover proofs of a remarkable variation in the character of the lavas.^ 

describirs Cretaceous tescheuites and diabases in Silesia {Tsrfiermak's Min. Miitheii. vii. 
(1885), p. 1.5). P. Choffat refers to Cenomanian emptions in Portugal {Joum. Seiencias 
Math. Phys. Xatui\ Lisbon, 1884). A. £. T.«agorio )ias found in the Crimea a series of 
sheets, dykes, and bosses, ranging from nevadites to basalts. 

^ For early and classical accounts of the Italian volcanic districts, see SpaUaniani's 
' Voyages dans les deux Siciles,' ai)d Breislak's ' Voyages Physiques et Lythologiques dams 
la Cani]ianie.' Consult also Mercalli's ' Vulcani, &c.,' and Johnston-La\is' 'South Italian 
Volcanoes,* already cited. 

'^ For a recent attempt to give a stratigraphical and geographical view of the distribntion 
of igneous rocks in Europe, see M. Bertrand, Bull. Str. GM. France^ xvi. (1888), p. 678. 

^ Tn some volcanoes {e.g. Teneriffe) the lower lavas arc heavier and more basic than the 
upi>er. 



SECT, i § 5 CAUSES OF VOLCANIC ACTION 263 

Various observers have noticed that volcanic rocks have succeeded 
each other in a certain order in different regions. Baron von Eichthofen 
deduced from observations in Europe and America a general sequence of 
volcanic succession, which he arranged in the following order : — 1. Pro- 
pylite; 2. Andesite ; 3. Trachyte; 4. Rhyolite; 5. Basalt^ This 
sequence he believed to be seldom or never complete in any one locality ; 
sometimes only one member of the series may be found ; but when two or 
more occur they follow, in his opinion, this sequence, basalt being every- 
where the latest of the series. The subject has been more recently 
discussed by M. Bertrand, who remarks that in Europe each of the great 
areas of plication has given rise to the formation of eruptive rocks of 
every composition and structure. He recognises a recurrence of the 
phenomena in successive geological periods, and speaks of a definite order 
of eruptions in the same series.^ 

The great volcanic series of Auvergne presents a marvellous succession 
of varied eruptions within a limited region during what was probably 
a single volcanic period. The first eruptions app>ear to have been 
basalts, and rocks of similar character reappeared again and again in later 
stages of the history, the intervening eruptions consisting of phonolites, 
trachytes, rhyolites, or andesites. The latest lavas were scoriaceous 
basalts.^ Among the later Palaeozoic volcanic eruptions of Britain a more 
definite and regular recurrence of rocks appears to be traceable. The 
earlier lavas of the Old Red Sandstone and Carboniferous series were 
generally either intermediate or basic, sometimes remarkably basic, while 
the late protrusions were decidedly acid. At the one end we find basalts 
or diabases and picrites, followed sometimes by copious outpourings of 
andesites, while at the other end come intrusions of felsites and quartz- 
porph3rries. Again, among the Tertiary lavas, the basalts of the great 
plateaux are pierced by bosses and dykes of granophyre and allied acid 
rocks. In these various examples the facts point to some gradual change 
in the composition of the subterranean magma during the lapse of a single 
volcanic period — a change in which there was a separation of basic 
constituents and the discharge of more basic lavas, leaving a more acid 
residuum to be erupted towards the end of the activity."* 

§ 5. Causes of Volcanic Action. 

The modus operandi whereby the internal heat of the globe manifests 
itself in volcanic action is a problem to which as yet no satisfactory 
solution has been found. Were this action merely an expression of the 
intensity of the heat, we might expect it to have manifested itself in a 
far more powerful manner in former periods, and to exhibit a regularity 
and continuity commensurate with the exceedingly slow diminution of 
the earth's temperature. But there is no geological evidence in favour 

' 'The Natural System of Volcanic Rocks,' Calif om, Acad, Set, 1868. 
» Bull. Soc. Giol. France^ xvl (1888), p. 611. 

* Carte Oiol. dHaili. France^ Feuille 166 (Clermont Ferrand). 

* Quart. Journ. OeoL Soc. vol. xhiii. (1892), p. 177. 



264 DYNAMICAL GEOLOGY book in parti 

of greater volcanic intensity in ancient than in more recent periods ; on 
the contrary, it may be doubted whether any of the Palaeozoic yolcanoes 
equalled in magnitude those of Tertiary and perhaps even post-Tertiary 
times. On the other hand, no feature of volcanic action is more con- 
spicuous than its spasmodic fitfulness.^ 

As physical considerations negative the idea of a comparatively thin 
crust, surmounting a molten interior whence volcanic energy might be 
derived {ante, p. 53), geologists have found themselves involved in great 
perplexity to explain volcanic phenomena, for the production of which a 
source of no great depth would seem to be necessary. Some have sup- 
posed the existence of pools or lakes of liquid lava lying beneath the 
crust, and at an inconsiderable depth from the surface. Others have 
appealed to the influence of the contraction of the earth's mass, assuming 
the contraction to be now greater in the outer than in the inner 
portions, and that the effect of this external contraction must be to 
squeeze out some of the internal molten matter through weak parts of 
the crust 2 

That volcanic action is one of the results of terrestrial contraction 
can hardly be doubted, though we are still without satisfactory data as 
to the connection between the cause and the effect. It will be observed 
that volcanoes occur chiefly in lines along the crests of terrestrial ridges. 
^^There is probably, therefore, a connection between the elevation of these 
ridges and the extravasation of molten rock at the surface. The forma- 
tion of continents and mountain-chains has already been referred to as 
prolmbly consequent on the subsidence and readjustment of the cool 
outer shell of the planet u])on the hotter and more rapidly contracting 
nucleus. Every such movement, by relieving pressure on regions below 
the axis of elevation, will tend to bring up molten rock nearer the 
surface, and thus to promote the formation and continued activity of 
volcanoes. 

The fissure-eniptions, wherein lava has risen in innumerable rents 
in the ground across the whole breadth of a country, and has been 
poured out at the surface over areas of many thousand square miles, 
flooding them sometimes to a depth of several thousand feet, undoubtedly 
prove that molten rock existed at some depth over a large extent 
of territory, and that by some means still unknown, it was forced out to 
the surface (aiifCy p. 256). In investigating this subject, it would be 
important to discover whether any evidence of great terrestrial crumpling 
or other movement of the crust can be ascertained to have taken place 
about the same geological period as a stupendous outpouring of lava — 

* Consult Dana, 'Characteristics of Volcanoes/ p. 15 et seq. Dutton, U. & Oeoi, JUp. 
1882-83, p. 183 et ««?. Prestwich. Proc. Roy. Soc. xlL (1886), p. 117. Uiwl, Jahrh, Qtol, 
Reichsanst. 1886, p. 315. 

^ Conlier, for example, calculated that a contraction of only a single millimetre (about 
^s^th of an inch) would suffice to force out to the surface lava enough for 500 eropUoiu. 
allowing 1 cubic kilometre (about 1300 million cubic yards) for each eruption. Prof. Ptest- 
wich invokes a slight contraction of the crust as the initial cause of volcanic action. Brit, 
Assoc, 1881, Sects, p. 610. 



SECT, i § 5 CAUSES OF VOLCANIC ACTION 265 

whether, for example, the great lava-fields of Idaho may have had any 
connection with contemporaneous flexure of the North American 
mountain-system, or whether the basalt-plateaux of Antrim, Scotland, 
Faroe, and Iceland may possibly have been in their origin sympathetic 
with the postrEocene upheaval of the Alps or other Tertiary movements 
in £urope. The most striking instance of an apparent connection between 
such terrestrial disturbances and volcanic phenomena is that supplied by 
the great semicircle of eruptions that sweeps from Central France by the 
£ifel, Hochgau, and Bohemia into Hungary, and which has been referred 
to the dislocations consequent on the upheaval of the Alps.^ 

In the ordinary phase of volcanic action, marked by the copious 
evolution of steam and the abundant production of dust, slags, and cinders, 
from one or more local vents, the main proximate cause of volcanic 
excitement is obviously the expansive force exerted by vapours dissolved 
in the molten magma from which lavas proceed. Whether and to what 
extent these vapours are parts of the aboriginal constitution of the 
earth's interior, or are derived by descent from the surface, is still an 
unsolved problem. The abundant occlusion of hydrogen in meteorites, 
and the capacity of many terrestrial substances, notably melted metals, 
to absorb large quantities of gases and vapours without chemical com- 
bination, and to emit them on cooling with eruptive phenomena, not 
unlike those of volcanoes, have led some observers to conclude that the 
gaseous ejections at volcanic vents are portions of the original con- 
stitution of the magma of the globe, and that to their escape the activity 
of volcanic vents is due. Prof. Tschermak in particular has advocated 
this opinion, and it is meeting with increasing acceptance.^ 

On the other hand, since so large a proportion of the vapour of active 
volcanoes consists of steam, many geologists have urged that this steam 
has in great measure been supplied by the descent of water from above 
ground. The floor of the sea and the beds of rivers and lakes are all 
leaky. Moreover, during volcanic eruptions and earthquakes, fissures no 
doubt open under the sea, as they do on land, and allow the oceanic water 
to find access to the interior.^ Again, rain sinking beneath the surface of 
the land, percolates down cracks and joints, and infiltrates through the 
very pores of the rocks. The presence of nitrogen among the gaseous 
discharges of volcanoes may indicate the decomposition of water containing 
atmospheric gases. The abundant sublimations of chlorides are such as 
might probably result from the decomposition of sea-water. To some 
extent surface-waters doubtless do reach the volcanic magma. 

* Sucss, 'AutlitzderErde,' i. p. 358, pi. iii. ; Jiilien, Anmuiiredu Club Alpin, 1879-80, 
p. 446 ; Michel-Levy, BuU, Soc. mol, France, xviii. (1890), pp. 690, 841. 

^ He has suggested that if 190 cubic kilometres, of the constitution of cast iron, be 
supposed to solidify annually, and to give off 50 times its volume of gases, it would 
suffice to maintain 20,000 active volcanoes. Sitz. Akad. Wissen, fftCTi, Ixxv. (1877), 
p. 151. Reyer (• Beitrag zur Physik der Eruptionen,' Vienna, 1877) advocates the same view. 

• Professor Moseley mentions that during a submarine eruption off Hawaii in 1877 "a 
fissure opened on the coast of that island, from a few inches to three feet broad, and in 
some places the water was seen pouring down the opening into the abyss below." ' Notes 
by a Naturalist on the " Challenger, '"p. 508. 



266 DYNAMICAL GEOLOGY book iii part i 



Whatever may be its source, we cannot doubt that to the enonnooB 
expansive force of superheated wat^r (or of its component gases, disso- 
ciated by the high temperatiu'e), in the molten magma at the roots of 
volcanoes, the explosions of a crater and the subsequent rise of a lava- 
column are mainly due. The water or gas dissolved in the lava is 
retained there by the enormous overlying pressure of the lava-colomu, 
but when the molten material is brought up to the surface the pressure 
is relieved and the water vaporizes and esca}3es. Where the relief is rapid 
the effect may be to froth up the lava into a pasty mass of pumice, while 
where it is sudden and extreme the escape of the water-vapour may be by 
an explosive discharge. 

It has been supposed that, somewhat like the reservoirs in which 
hot water and steam accumulate under geysers, re8er\'oirs of molten rock 
receive a constant influx of water from the surface, which cannot escape 
by other channels, but is absorbed by the internal magma at an 
enormously high temperature and under vast pressure. In the course of 
time, the materials filling up the chimney are unable to withstand 
the upward expansion of this imprisoned vapour or water-substance, so 
that, after some premonitory rumblings, the whole opposing mass is blown 
out, and the vapour escapes in the well-known masses of cloud. Mean- 
while, the removal of the overlying column relieves the pressure on the 
lava underneath, saturated with vapours or superheated water. Hus 
lava therefore begins to rise in the furmel until it forces its way through 
some weak i>art of the cone, or pours over the top of the crater. 
After a time, the vapour being expended, the energy of the volcano 
ceases, and there comes a variable period of repose, until a renewal of the 
same phenomena brings on another einiption. By such successive 
paroxysms, the forms of the internal reservoirs and tunnels may be 
changed ; new spaces for the accumulati<jn of superheated water being 
opened, whence in time fresh volcanic vents issue, while the old ones 
gradually die out. 

An obvious objection to this explanation is the difficulty of con- 
ceiving that water should descend at all against the expansive forc^ 
within. But Daubr<^e's experiments have shown that, owing to capil- 
larity, water may permeate rocks against a high counter-pressure of 
steam on the further side, and that so long as the water is supplied, 
whether by minute fissures or through pores of the rocks, it may, under 
pressiu*e of its own superincumbent column, make its way into highly 
heated regions.^ Experience in deep mines, however, rather goes to show 
that the permeation of water through the pores of rocks gets feebler as 
we descend. 

Keference may be made here to a theory of volcanic action in which 
the influence of terrestrial contraction as the grand source of volcanic 
energy was insisted upon by the late Mr. Mallet.^ He maintained that 



1 



Daubrt^e, * Geologie Ex penmen tale,' p. 274 (criticised adversely by Fisher, * Physics of 
Earth's Crust,' 2iid ed. p. 144). Tscherniuk,' cit^d on previous page. Reyer, * Beitrag nir 
Physik der Eruptionen,' § L 

' Phii. Trans. 1873. See also Daubree's experimental deterrolDation of the 



SECT, i § 5 CAUSES OF VOLCANIC ACTION 267 

all the present manifestations of hypogene action are due directly to the 
more rapid contraction of the hotter internal mass of the earth and the 
consequent crushing in of the outer cooler shell. He pointed to the 
admitted difficulties in the way of connecting volcanic phenomena with 
the existence of internal lakes of liquid matter, or of a central ocean of 
molten rock. Observations made by him, on the effects of the earth- 
quake shocks accompanying the volcanic eruptions of Vesuvius and of 
Etna, showed that the focus of disturbance could not be more than a 
few miles deep; that, in relation to the general mass of the globe, it 
was quite superficial, and could not possibly have lain under a crust of 
800 miles or upwards in thickness. The occurrence of volcanoes in lines, 
and especially along some of the great mountain-chains of the planet, was 
likewise dwelt upon by him as a fact not satisfactorily explicable on any 
previous hypothesis of volcanic energy. But he contended that all these 
difficulties disappear when once the simple idea of cooling and contraction 
is adequately realised. " The secular cooling of the globe," he remarks, 
" is always going on, though in a very slowly descending ratio. Contrac- 
tion is therefore constantly providing a store of energy to be expended 
in crushing parts of the crust, and through that providing for the volcanic 
heat. But the crushing itself does not take place with uniformity ; it 
necessarily acts per solium after accumulated pressure has reached the 
necessary amount at a given point, where some of the pressed mass, un- 
equally pressed as we must assume it, gives way, and is succeeded 
perhaps by a time of repose, or by the transfer of the crushing action 
elsewhere to some weaker point. Hence, though the magazine of volcanic 
energy is being constantly and steadily replenished by secular cooling, 
the effects are intermittent." He offered an experimental proof of the 
sufficiency of the store of heat produced by this internal crushing to cause 
all the phenomena of existing volcanoes.^ The slight comparative depth 
of the volcanic foci, their linear arrangement, and their occurrence along 
lines of dominant elevation become, he contended, intelligible under this 
hypothesis. For since the crushing in of the crust may occur at any 
depth, the volcanic sources may vary in (lepth indefinitely ; and as the 
crushing will take place chiefly along lines of weakness in the crust, it is 
precisely in such lines that crumpled mountain-ridges and volcanic 
funnels should appear. Moreover, by this explanation its author sought to 
harmonise the discordant observations regarding variations in the rate of 
increase of temperature downward within the earth, which have already 
been cited and referred to unequal conductivity in the crust (p. 51). He 
pointed out that in some parts of the crust the crushing must be much 
greater than in other parts ; and since the heat " is directly proportionate 

quantity of heat evolved by the iuteraal crushing of rocks. 'Geologie Experimentale,' 
p. 448. For an adverse criticism of Mallet's view, see Fisher, op. cit. chap. xxii. 

* The elaborate and careful exj)erimeutal researches of this observer will reward attentive 
perusal. Mallet estimates from experiment the amount of heat given out by the crushing of 
different rocks (syenite, granite, sandstone, slate, limestone), and concludes that a cubic mile 
of the crust taken at the mean density would, if crushed into powder, give out heat enough 
to melt nearly 3} cubic miles of similar rock, assuming the melting-point to be 2000** Fahr. 



268 DYNAMICAL GEOLOGY book m paw i 

to the local tangential pressure which produces the crushing and the 
resistance thereto," it niay vary indefinitely up to actual fusioiL So long 
as the crushed rock remains out of reach of a sufficient access of 
subterranean water, there would, of course, be no disturbance. 
But if, through the weaker parts, water enough should descend 
and \ye absorbed by the intensely hot crashed mass, it would be raised to 
a very high temperature, and, on sufficient diminution of pressure, 
would flash into steam and produce the commotion of a volcanic 
eruption. 

This ingenious theory recjuires the operation of sudden and violent 
movements, or at least that the heat generated by the crushing should 
be more than can be immediately conducted away through the crust 
Were the crushing slow and equable, the heat developed by it might be 
so tranquilly dissipated that the temi^eraturo of the ciiist would not be 
sensibly affected in the process, or not to such an extent as to cause any 
appreciable molecular rearrangement of the particles of the rocks. But 
an amount of internal crushing insufficient to generate volcanic action 
may have been accompanied by such an elevation of temperature as to 
induce important changes in the structure of rocks, such as are embraced 
under the t^rm " metamorphic." 

There is, indeed, strong e\'idence that, among the consequences arising 
from the secular contraction of the globe, masses of sedimentary strata, 
many thousands of feet in thickness, have been crumpled and crushed, 
and that the crumpling has often l)een accompanied by such an amount 
of heat and evolution of chemical acti>dty as to produce an interchange 
and rearrangement of the elements of the rocks, — this change sometimes 
advancing perhaps to the point of actual fusion. (See posiea, p. 298, and 
Book IV. Part VIII.) There is reason to l^elieve that some at least of 
these periods of intense terrestrial disturbance have been followed by 
periods of prolonged volcanic action in the disturl>cd areas. Mr. Mallet's 
theory is thus, to some extent, supported by independent geological testi- 
mony. The existence, however, of large reservoirs of fused rock, at a 
comi>aratively small depth beneath the surface, may be conceived as prob- 
able, a|>art from the effects of crushing. The connection of volcanoes 
with lines of elevation, and consequent weakness in the earth's crust, i« 
what might have been anticipated on the view that the nucleus, though 
practiciilly solid, is at such a temperatui*e and pressure that any diminu- 
tion of the pressure, by cormgation of the cinist or otherwise, will cause 
the subjacent portion of the nucleus to melt. Along lines of elevation 
the pressure is relieved, and consequent melting may take place. On 
these lines of weakness and fracture, therefore, the conditions for volcanic 
excitement may be conceived to be best developed, whether arising from 
the explosive energy of water dissolved in the magma or from water 
descending to the intensely heated materials underneath the crust The 
periodicity of eruptions may thus depend upon the length of time 
required for the storing up of sufficient steam, and on the amount of 
resistance in the crust to be overcome. In some volcanoes, the intervals 
of activity, like those of many geysers, return with considerable regularity. 



8BCT. i § 6 CAUSES OF VOLCANIC ACTION 269 



In other cases, the shattering of the crust, or the upwelling of vast masses 
of lava, or the closing of subterranean passages for the descending water, 
or other causes may vary the conditions so much, from time to time, that 
the eruptions follow each other at very unequal periods, and with very 
discrepant energy. Each great outburst exhausts for a while the vigour 
of the volcano, and an interval is needed for the renewed accumulation 
of vapour. 

But beside the mechanism by which volcanic eruptions are produced, 
further problems are presented by the varieties of materials ejected, by 
the differences which these exhibit at neighbouring vents, even some- 
times in successive eruptions from the same vent, by the alternation or 
recurrence of lavas from basic to acid in the continuance of a single 
volcanic period, and by the repetition of a similar cycle in successive 
periods. Observations are yet needed from a larger number of ancient 
volcanic districts and in greater detail, before these problems can be 
satisfactorily discussed and solved. It is obvious that in such a great 
series of eruptions as that of Central France, where over a comparatively 
limited area an alternation of basic and acid lavas has been many times 
repeated, the subterranean magma must have undergone a succession of 
changes in composition. Perhaps a definite cycle of such alternations may 
be made out. The sequence from basic to acid protrusions, observable 
among the British Palaeozoic volcanic rocks, is suggestive of a separation 
of the more basic constituents of the magma with consequent increasing 
acidity of the residue. The earliest lavas mark the more basic condition 
of the magma, while the latest felsite and quartz-porphyry intrusions 
show its impoverishment in bases at the close of a volcanic period. 
During the interval before the next period the magma had in some way 
been renewed, for when eruptions began anew they were once more 
basic. But by the close of the volcanic activity the magma had again 
lost a large proportion of its basic constituents and had become 
decidedly acid. 

Reference has already (p. 61) been made to the speculation of 
Durocher as to the existence within the crust of an upper siliceous layer 
with a mean of 71 per cent of silica, and a lower basic layer with about 51 
per cent of silica. Bunsen also came to the conclusion that volcanic rocks 
are mixtures of two original normal magmas — the normal trachytic (with 
a mean of 76*67 silica), and the normal pyroxenic (with a mean of 47*48 
silica). The varying proportions in which these two original magmas 
have been combined are, in Bunsen's view, the cause of the differences 
of volcanic rocks. We may conceive these two layers to be superposed 
upon each other, according to relative densities, and the composition 
of the last material erupted at the surface to depend upon the depth 
from which it has been derived.^ The earliest explosions may be 
supposed to have taken place usually from the upper lighter and more 

* See R. Bunsen, Pogff. Anti. Ixiii. (1851), p. 204; Sartorius von Waltershauseu, 
'Sicilien und Island,' p. 416 ; Reyer. ' Beitragzur Physik der Eruptionen,' iiL Scrope had 
long before snggesteil a classification of volcanic rocks into Trachyte, Greystone, and 
Basalt, Joum. Sdence^ xxi. 



270 DYNAMICAL GEOLOGY book iii 



siliceous layer, and the lavas ejected would consequently be in general 
acid, while later eruptions, reaching down to deeper and heavier zonet 
of the magma, brought up such basic lavas as basalt. Certainly the 
general similarity of the volcanic rocks all over the globe would appear 
to prove that there must be considerable uniformity of composition in 
the zones of intensely hot material from which volcanic rocks are 
derived.^ 

Many difRculties, however, remain yet to be explained before our 
knowledge of volcanic action can be regarded as more than rudimentary. 
In Book IV. Part VII. a description is given of the part volcanic 
rocks have played in building up what we see of the earth's crust, and 
the student will there find other illustrations of facts and deductions 
which have been given in the previous pages. 

Section ii. Earthquakes.^ 

By the more delicate methods of observation which have been 
invented in recent years, it has been ascertained that the ground beneath 
our feet is apparently everywhere subject to continual slight tremors and 
to minute pulsations of longer duration. The old expression " terra 
firma " is not only not strictly true, but in the light of modem research 
seems singularly inappropriate. liapid changes of temperature and 
atmospheric pressure, the fall of a shower of rain, the patter of birds' feet, 
and still more the tread of larger animals, produce tremors of the ground 
which, though exceedingly minute, are capable of being made clearly 
audible by means of the microphone and visible by means of the galvano- 
meter. Some tremors of varying intensity and apparently of irregular 
occurrence, may be due to minute movements or displacements in the 
crust of the earth. Less easily traceable are the slow pulsations of the 
crust, which in many cases are periodic, and may depend on such causes 
as the diurnal oscillation of the thermal or barometric conditions of the 
atmosphere, the rise and fall of the tides, <fec. So numerous and well 
marked are these tremors and pulsjitions, that the delicate observations 

^ In the memoir by Captain Dutton, cited iu a ])revioiis note, the hypothesis is main' 
tained that the order of ai>pearance of the lavas is determined by their relative density 
and fusibility, the most basic and heaviest, though most easily fused, requiring fhe 
highest temperature to diminish their density to such an extent as to permit them to be 
enipted. 

- On the phenomena of earthquakes consult Mallet, Brit, Asuoc. 1847, part ii. p. 80; 
1850, p. 1 ; 1851, p. 272 ; 1852, p. 1 ; 1858, p. 1 ; 1861, p. 201 ; 'The Great Neapolitan 
Earthquake of 1857,' 2 vols., 1862 ; D. Milne, Iklin. New Phil. Journ, zxzi.-xxzvi. ; A. 
Perrey, MSm. Cvuronn. Bruxelles, xviii. (1844), Comptes renduSf Iii. p. 146 ; Otto Volger, 
* Untersuchungen iiber die Phanomeue der Erdbeben in der Schwelz,' Gk>tha, 1857-8; 
Z. JJeiitsch. Oed. Ges, xiii. p. 667 ; R. Falb, * Grundziige einer Theorie der Erdbeben 
und Vulkanensausbriiche,' Graz, 1871 ; *Gedankeu und Studien ttber den Vnlkanismns, 
&c.,' 1874: Pfaflf, *Allgemeine Geologie als exactc Wissenschaft,* I^ipzig, 1873, p^ 224. 
Rcconls of observed earthquakes will be found in the memoirs of Mallet and Perrey ; also 
i>i pauers by Fuchs iu Seues Jahrb. 1865-1871, and in Tschermak's Mineradog, MiUheSi- 
1873 and subsequent years. See also Schmidts ' Studien iiber Erdbeben,' 2nd edit 



'ipauers 
'^fc8 



PART I SECT, ii EARTHQUAKES 271 



which were set on foot to determine the lunar disturbance of gravity had 
to be abandoned, for it was found that the minute movements sought for 
were wholly eclipsed by these earth tremors.^ "* 

The term Earthquake denotes any natural subterranean concussion, 
varying from such slight tremors as to be hardly perceptible up to 
severe shocks, by which houses are levelled, rocks dislocated, landslips 
precipitated, and many human lives destroyed. The phenomena are 
analogous to the shock communicated to the ground by explosions of 
mines or powder-works. They may be most intelligibly considered as 
wave-like undulations propagated through the solid crust of the earth. 
In Mr. Mallet's language, an earthquake may be defined as " the transit 
of a wave of elastic compression, or of a succession of these, in parallel 
or intersecting lines through the solid substance and surface of the 
disturbed country." Mr. Milne has since remarked that the disturbance 
may also be due to the transit of waves of elastic distortion. The 
passage of the wave of shock constitutes the real earthquake. 

Besides the wave of shock transmitted through the solid crust, waves 
su'e also propagated through the air, and, where the site of the impulse 
is not too remote, through the ocean. Earthquakes originating under the 
sea are numerous and specially destructive in their effects. They illustrate 
well the three kinds of waves associated with the progress of an earth- 
quake. These are, 1st, The true earth- wave through the earth's crust ; 
2nd, a wave propagated through the air, to which the characteristic sounds 
of rolling waggons, distant thunder, bellowing oxen, &c., are due ; 3rd, 
Two sea-waves, one of which travels on the back of the earth-wave and 
reaches the land with it, producing no sensible effect on shore ; the other 
EUi enormous low swell, caused by the first sudden blow of the earth- wave, 
but travelling at a much slower rate, and reaching land often several 
hours after the earthquake has arrived. 

Amplitude of earth -movements. — The popular conception of the 

1879; *Stu(lien iiber Vulkane und Erdbeben,' 1881 ; Dieffenbach, NeuesJahrb. 1872, p. 155 ; 
M. S. di Rossi, ' La Meteorologia Eudogena,' 2 vols. 1879 and 1882 ; M. Gatta, • L'ltalia, 
in Tolcanl e terremoti,' 1882; J. Milne, * Earthquakes and other Earth -movements,' 
1886, and his beautifuUy illustrated volume on the Japan Karthquake of October 1891. 
6. Mercalli, in bis ' Yulcaui e Fenomeni Vulcanici in Italia ' (1883), gives an account of the 
Italian earthquakes from 1450 b.c. to 1881 a.d. ; he separately describes the great Ischian 
earthquake of 1883 : * L'Isola d'Ischia,' Milan, 1884. Much interesting information will be 
found in the BuUeiino del Vulcanismo ItalianOt which began to be published in 1874 ; also 
in the Transactions of the Setsmological Society of Japan — a society instituted in the year 
1880 for the investigation of earthquake phenomena, especially in Japan, where they are of 
freqnent occurrence. Others papers are quoted in the following pages. 

* A. d'Abbadie, * Etudes sur la verticale,* 1872. Plantamour, Comptes rend. June 1878, 
February 1881 ; Archives Sciences Phys. Nat. Geneva, iu p. 641 ; v. p. 97 ; vii. p. 601 ; 
viiL p. 551 ; x. p. 616 ; xii. (1884), p. 388. G. H. Darwin, BrU. Assoc. 1882, p. 95. In 
this paper Prof. Darwin discusses the amount of disturbance of the vertical near the coasts 
of continents, caused by the rise and fall of the tide. J. Milne, Trans. SeisnidtHfical Sac. 
Japan, vi. (1888), p. 1 ; Oecl. Mag. 1882, p. 482 ; Naiurey xxvi. p. 125. The numerous 
Dbaervations made by Rossi in Italy are summarised by G. Mercalli in his work cited above, 
p. 332. 



272 DYNAMICAL GEOLOGY book m 

extent to which the ground moves to and fro or up and down during an 
earthquake is a great exaggeration of the truth. As the result of very 
careful me^urement ^ith delicate instnunents, there appears to be reason 
to believe that the horizontal motion at the time of a small earthquake 
is usually only the fraction of a millimetre, and seldom exceeds three or 
four millimetres. A\Tien the motion rises to five or six millimetres brick 
and stone chimneys are shattered. Yet even with such an intensity of 
shock a person walking in an open place might be quite unconscious of 
any perceptible movement of the ground. The vertical motion also 
appears to be exceedingly small. ^ 

Velocity. — Experiments have been made to determine the velocity 
of the earth -wave, and its variation with the natm*e of the material 
through which it is propagated. Mi*. Mallet found that the shock 
produced by the explosion of gunpowder travelled at the rate per second 
of 825 feet in sand; 1088 feet in schists, slates, and quartzites ; 1306 
feet in friable g!-anite ; and 1664 feet in solid granite. General Abbot, 
by observing the effects of the explosion of dynamite and gunpowder, 
found the velocity of transmission of the shock to vary from 1240 to 
8800 feet per second, and to be gi'catest where the shock is most violent.* 
Observations of the time at which an earthquake has successively visited 
the different places on its track have shown similar variations in the rate 
of movement. Thus in the Calabrian earthquake of 1857, the wave of 
shock varied from 658 to 989 feet per second, the mean rate being 789 
feet. The earthquake at Vi<Vge in 1855 was estimated to have travelled 
northwards towards Sti-asburg at the rate of 2861 feet per second, and 
southwards towards Turin at a mtc of 1398 feet, or loss than half the 
northern speed. The earthquake of 7th October 1874, in northern Italy, 
travelled at rates varying from 273 to 874 feet per second. That of 
12th March 1873 showed a velocity per second of 2734 feet between 
Ragusa and Venice; 4101 feet from Spoleto to Venice ; 601 feet from 
Perugia to Orvicto ; 1640 feet from Perugia to Ancona; and 1640 (or 
2188) feet from Perugia to Rome. The rate of the central £ui*opean 
earthquake of 1872 was estimated to have been 2433 feet, that of 
Herzogenrath, June 24, 1877, 1555 feet, that of an earthquake at 
Travancore, in Southern Hindustan, 656 feet in a second.* The most 
accurate metvsuremcnts and computiitions of the velocity of earthquake 

^ Milne. ' Rirth quakes,' pp. 75, 76. An ingenious nicxlel in wire has been made by Prof. 
Sekiya to illustrate the highly complex i>nth pursued by a particle on the surface of the 
jjrouujl duriug an earthquake at Tokio, Japan, on 15th January 1887. 

'^ Avfi'. Jmirn, Sci. xv. (1878). l*rof. J. Milne, exj)erimenting in Jaium, has likewise 
ascertaineil that a close relation exists between the initial violence of the shock and the 
velocity of propagation, and that there is a progressive diminution in speed as the ware of 
sho<;k travels outward from tlic centre of disturbance. 'Earthquakes,' p. 65. 

» K. von Seebach, *Das M itteldeutsche Erdbel)en von 6 Marz, 1872,' Leipzig, 1873. 
Hiifer, SU^Jb. Akad. Wien, Dec. 1876 ; A. von Lasaulx, 'Das Erdbeben von Herzogenrath, 
22ud Oct. 1873,' Bonn, 1874. *Das Erdbeben von Herzogenrath, 24 Juni, 1877/ Bonn, 
1878. G. C. Laube, on Earth<iuake of 31st January 1883, at Trautenau, Jahrb. Otol, lUidu. 
1883, p. 331. 11. Credner on the Earthquakes of the Erzgebirge and Vogtland from 1878 
to 1884, ZeitHch. fiir Xattirtnss. vol. Ivii. (1884). F. Wahner, on Agram earthquake of 9 



PART I SECT, ii EARTHQUAKES 273 

movements are probably those made by Prof. J. Milne and his associates 
in Japan. The rates of movement diu-ing the Tokio earthquake of 
25th October 1881 are estimated to have ranged between 4000 and 
9000 feet per second. As the result of prolonged observation, Prof. 
Milne concludes that "different earthquakes, although they may travel 
across the same country, have very variable velocities, varying between 
several hundreds and several thousands of feet per second ; that the 
same earthquake travels more quickly across districts near to its origin 
than it does across districts which are far removed ; and that the greater 
the intensity of the shock, the greater is the velocity." ^ 

Duration. — The niunber of shocks in an earthquake varies in- 
definitely, as well as the length of the intervals between them. Some- 
times the whole earthquake only lasts a few seconds : thus the city of 
Caracas, with its fine churches and 10,000 of its inhabitants, was 
destroyed in about half-a- minute ; Lisbon was overthrown in five 
minutes. But a succession of shocks of varying intensity may continue 
for days, weeks, or months. The Calabrian earthquake, which began in 
February 1783, was continued by repeated shocks for nearly four years 
until the end of 1786. 

Modifying influence of geologrical structure. — In its passage 
through the solid terrestrial crust from the focus of origin, the earth- 
wave must be liable to continual deflections and delays, from the varying 
geological structure of the rocks. To this cause, no doubt, must be in 
large measure ascribed the marked differences in the rate of propagation 
of the same earthquake in different directions. The wave of disturbance, 
as it passes from one kind of rock to another, and encounters materials 
of very different elasticity, or as it meets with joints, dislocations, and 
curvatures in the same rock, must be liable to manifold changes alike in 
rate and in direction of movement. Even at the surface, one effect of 
differences of material may be seen in the apparently capricious demo- 
lition of certain quarters of a city, while others are left comi>aratively 
scatheless. In such cases, it has often been found that buildings erected 
on loose inelastic foundations, such as sand and clay, are more liable to 
destruction than those placed upon solid rock. In illustration of this 
statement the accompanying plan (Fig. 71) of Port Royal, Jamaica, was 
given by De la Beche - to show that the portions of the town which did 
not disappear during the earthquake of 1692 were built upon solid white 
limestone, while the parts built on sand were shaken to pieces.^ 

It has been observed that an earthquake shock ^vill pass under a 
limited area without disturbing it, while the region all around has been 
affected, as if there were some superficial stratum protected from the 

Nov. 1880, SUz, Akad. Wiefif Ixxxviii. (1883), p. 15. Di Rossi, ' Meteorologia Endogena,* 
L p. 306 ; P. Serpieri, InstitiUo LombardOj 1873. 

* * Earthquakes,' p. 94. 

* 'Geological Observer,' p. 426. 

' The opposite effect has been observed on tlie island of Ischia, the houses built on loose 
subsoil generaUy havjng suffered much less than the others. There appears, indeed, to be a 
considerable conflict of testimony on this subject. See Milne, * Earthquakes,' p. 130. 

T 



374 nVNAMICAL GEOLUGY book ui 

earth-wave. Humboldt cited a caae wheie miners were driven up from 
below-gi-ound by earthquake shocks not i>erce])tible at the surface, and on 
the other hand, an instance where they expei-ieiiced no Bensation (rf ui 
efiithquake which shook the surface with considerable \-iolence.' Sudi 




Fii:. ri.— I-laD i>r Fort RoyBl, Juiiuk's. nlxin-iMx tlic effrctH ut Uii' Eiutbcia*kE uT lnoi (a> 

facts bring impressively l^efure the mind the extent to which the coiine 
of the earth-wave nuist be modified by geological structure. In some 
instances, the shuck extends outwards from a common centre, so that « 
series of concentric circles may be drawn round the foais, each of which 
will denote a cei-tain a]>pi-oximately unifonu intensity of shock ("coaeinnic 
lines " of Mallet), this intensity, of course, diminishing with distance from 
the focus. The C'alabriau eai-thquake of 1857 and that of Central Europe 
in 1872, may ))o taken in itiuiitration of thii> central type. In other 
cases, however, the earth<|iiake travels chiefly along a certain Iwind or 
zone (particularly along the flanks of a niouiUain-chain) without advanc- 
ing far from it latenilly. This tyi)e of linear earthtpuike is exemplified 
by the freijuent shocks which ti-averse Chili, Pern, and Ecuador, between 
the line of the Andes an<l the Pacific coaat.- 

Extent of country afTected.— The area shaken by an earthquake 
ViH-icH with the intensity of the shoik, from a mere local ti-act where a 
slight tremor has lieen experienced, up to such catastrophes as that of 
Lisbon in 17."iij, which, besides convulsing the Portuguese coasts, extended 
into the north of Africa on the one hand and to Scandinavia on the other, 
and was even felt as far as the east of Xoith America. Humboldt com- 
puted that the urea shaken by this great earthquake was foiu* times 
greater than that of the whole of Eurojie. The South Amencan earth- 
({uakes are remarkable for the gi-eat distances to which their effects 
extend in a linear directioiu Thus the strip of country in Peru and 
Ecuador severely shaken hy the earthquake of I8C8, had a length of 
aOOO miles. 

' 'CoiCHOs,' Art. Earlhquakti. 

■ For a lint oF Peruvian eartliqiiakcs from a.D. 1570 ta 1875, Ke Oeograph. Mag. i<r. 
(1877), ji. 208. The enrtliquake of » Slny, 1877, at Iqiiique, aud ita oc^n-wsve ue dsKriM 
by E. GeiniU, JVoki Ad. Ac. Cat. LeepoIJ. Car. xl. <187S), pp. 383-444. 



PABTisBCT. ii EARTHQUAKES 276 

Depth of source, — Accordii^ to Mallet's observations, over the 
centre of origin the shock is felt as a vertical up-and-down movement 
{SeismK vetiicat) ; while, receding from this centre in any direction, 
it is felt as an undulatory movement, and comes up more and more 
obliquely. The angle of emergence, as he termed it, was obtained by him 
by taking the mean of observations of the rente and displacements of 




walls and buildings. In Fig. 72, for example, the wall there represented 
has been rent by an earthquake which emerged to the surface in the path 
marked by the arrow. 

By observations of this nature. Mallet estimated the approximate 
depth of origin of an earthquake. -Let Fig. 73, for example, represent 
a portion of the eai-th's crust in which at a an caithquake arises. The 




wave of shock will travel outwards in successive spherical shells. At 
the point e it yn\l be felt as a vertical movement, and loose objects, 
such as paving-stones, may be jerked up into the air, and descend bottom 
uppermost on their previous sites. At rf, however, the wave will emerge 
at a lower angle, and will give rise to an undulation of the ground, and 
the oscillation of objects projecting above the surface. In rent buildings, 
the fissures will be on the whole perpendicular to the path of emergence. 
By a series of observations made at different points, as at ? and /, a 
number of angles are obtained, and the point where the various lines 
cut the vertical (a) will mark the area of origin of the shock. By this 






276 PYX A MICA L GEOLOGY bookih 



means, Mallet computed that the depth at which the impulse of the 
Calabrian earthquake of 1857 was given was about five miles. As the 
general result of his inquiries, he concluded that, on the whole, the origin 
of carthfjuakes must be sought in comparatively superficial parts of the 
crust, probably never exceeding a depth of 30 geographical mileB. 
Following another method of calculation, Von Seebach computed tliat 
the earthquake which affected Central Euroi)e in 1872 originated at a 
depth of 9 '6 geographical miles; that of Belluno in the same year wu 
estimated by Hofer to have had its source rather more than 4 miles deep; 
while that of Hcrzogenrath in 1873 was placed by Von Lasaulx at i 
depth of a1x)ut 14^ miles, and that of 1877 in the same region at about 
14 miles. ^ 

Geologrieal EflTects. — These are dependent not only on the strsngli 
of the concussion but on the structure of the ground, and on the site of 
the disturbance, whether underneath land or sea. They include ^^IwmgM 
superinduced on the surface of the land, on terrestrial and oceanic wiUl% 
and on the relative levels of land and sea. 

1. Effects upon the soil and general surface of a country. — 
The earth- wave or wave of shock underneath a country may timve ii e a 
wide region and atfect it violently at the time, without leaving permaiient 
traces of its i>assage. Blocks of rock, however, already disengaged from 
their jxirent masses on declivities, may be rolled down into the vmUeyn 
liiindslips are ])roduced, which may give rise to considerable sufasequent 
changes of drainage. In some instances, the siuiaces of solid rocks are 
shattered as if by ginipowder, as was i>articularly noticed to have taken 
place among the Primary* rocks in the Concepcion earthquake of 1835.^ 
It has nfteu been observed also that the soil is rent by fissures vhidi 
vary in size from mere cracks, like those due to desicciition, up to chasms 
a mile or more in length and 200 feet or more in depth. Permanent 
ni<Hliticatii»ns of the landscai>e may thus be produced. Trees are thrown 
down, and buried, wholly or in jwrt, in the rentes. These superficial 
ctlects may, indeoil, be soon ettaced by the levelling jwwer of the atmo- 
spheric. \\'here, however, the chasms are wide and deep enough to 
intercept rivulets, or to serve as channels for heavy rain-torrents, they 
are sometime** further excavated, so as to become graduidly enlarged into 
ravines and valleys i^s has hapiKMied in the case of renU caused by the 
earth«iuakos of ISll-l-, in the Mississippi valley. In the earthquake 
whii'h sluH^k the Si^uth Island i*f New Zealand in 1848, a fissure was 
formed, averairiuir IS inches in width and traceable for a distance of 60 
miles jKirallel to the axis of the adjacent mountain-chain. The subsequent 
eartln|uake of L^T*.""*. in the same region, gjive rise to a fnicture which 
could l»e tractxl along the Ivise of a line of cliff for a distance of about 
00 miles. l>r. i.>klham has desi-rilnxi a remarfcible series of fissurings 
which ran jKU-allel with the river oi Calhar, Eastern British India, 

■ Siv i..'.iH'r'» ^^y H -hr .uul A, von Uln^iuIx. i:;u»»l i xi y. 1*72. For an acconnt of the 
\.iri«v;< n;r:]uM> i*::'.vli\\otl i;i «>:iii;:i:iiii: iho iUj»:h of orijiin of earthquakes, see Milne'f 
• H.ir:::,|U.iki>/ iliaj'trVN x. aiiil \i. iVnsul: «l>o ::se T:\:r.s. Sdsmolog, Soc Japan. 

- D.iiwra. "Journal of Keseaiv-ho," 154.\ ]», 30?. 



CI. ii 



EARTHQUAKES 



277 



raryjng with it to every point of the compass and traceable for 100 
niles.' The great Japanese earthquake of 26tb October 1891 gave rise 
jO Bome remarkable fractures of the ground, in one of which one eide 
iras placed pei-raanently at a different level from the other (Fig. 74). 




fig. 7<.-Ft*iur« 



It«m&rkable circulai* cavities have 1>een noticed in Ciilabria mid 
ilsewhere, formed in the ground during the passjige of the earth-wave. 
!n many cases, these holes serve as funnels of escape for an abundant 
luch&rgo of water, so that when the disturbance ceases they ap[)ear an 
x>ole. They are beliei-ed to be ttiused by the sudden collapse of aul)- 
crranean water-channels and the conse<]uent forcible ejection of the water 
o the surface. Besiiles water, discharges of various gases and vapours, 
lometimes combustible, have Iwen noted .it the fissures formed during 
tarthquakes. 

2. Effects upon terrestrial waters.^ — Springs are temporarily 
ifTected by earthquake movements, becoming greater or smaller in 
volume, sometimes muddy or discoloured, and sometimes increasing 
n temperature. Brooks and rivers have been observed to flow with an 
ntemipted course, incre;tsing or diminishing in size, stopping in their 
low so as to leave their chaimels dry, and then rolling forward with 
ncreased rapidity. Lakes are still more sensitive. Their waters 
wcasionally rise and fall for several hours, even at a distance of many 
lundred miles from the centre of disturlmncc. Thus, on the day of the 

■ Q. J. Otol. ,4^. xxviii. ]:. -.'57. For a catalogue of Inilian Eiuthitiiiikes ilown to tlm 
ltd Of ISeS, Ke T. OUllinm, Mem. f;r.il. .•inn: India, lii. imrt -2. 
' Klnge, StHttJukrb. 18B1, ].. 777. 



278 nYXAMICAL GEOLOGY BOOKm 

great Lisbon earthquake, many of the lakes of central and north-western 
Europe were so aflected as to maintain a succession of waves rising to a 
height of 2 or 3 feet above their usual level. Cases, however, have been 
observed where, owing to excessive subterranean movement, lakes have 
been emptied of their contents and their beds have been left permanentlj 
dry. On the other hand, areas of dry ground have been depressed, and 
have become the sites of new lakes. 

Some of the most important changes in the fresh water of a region, 
however, are produced by the fall of masses of rock and earth, which, 
by damming up a stream, may so arrest its water as to form a lake. If 
the barrier be of sufRcient strength, the lake will be permanent ; thou^ 
from the usually loose, incoherent character of its materials, the dam 
thrown iicross the pathway of a stream runs a great risk of being 
undermined by the percolating water. A sudden giving way of tfce 
barrier allows the confined water to nish >\'ith great violence down 
the valley, and to produce perhaps tenfold more havoc there than may 
have been caused by the original earthquake. When a landslip is d 
sutficient dimensions to divert a stream from its previous course, the 
new channel thus taken may become permanent, and a valley may be 
cut out or ^Wdcned. 

3. Effects upon the sea. — The gi*eat sea- wave propagated outward 
from the centre of a sub-oceanic earthquake and reaching the land after 
the earth-wave has arrived there, gives rise to much destruction along the 
maritime parts of the disturbed region. AVhen it approaches a low shcve, 
the littoral waters retreat seawards, sucked up, as it were, by the advano- 
iug wall of water, which, reaching a height of sometimes 60 feet or more, 
rushes over the bare beach and sweeps inland, carrying with it everything 
which it can dislodge and bear away. Loose blocks of rock are thus 
lifted to a considerable distance from their former position, and left at a 
higher level. Deposits of sand, gravel, an<l other superficial accumula- 
tions are torn up and swept away, while the sm*face of the country, as far 
as the limit reached by the wave, is strewn with debris. If the 
district has been already shattered by the passage of the earth- 
wave, the advent of the great sea-wave augments and completes the 
devastation. The havoc caused by the Lisbon earthquake of 1755, and 
by that of Peni and Ecuador in 1868, was much aggi^vated by the co- 
oi)eration of the oceanic wave. Where the wave breaks on land rising 
out of deep water little damage may be done. 

4. Permanent changes of level. — It has been observed, after the 
pasirage of an earthquake, that the level of the disturbed country has 
sometimes been changed. Thus after the terrible earthquake of 19th 
November 1822, the coast of Chili, for a long distance, was found to 
have risen from 3 to 4 feet, so that along shore, littoral shells were 
exposed still adhering to the rocks, amid multitudes of dead fish. The 
same coast-line has been further upraised by subsequent earthquake 
shocks. • On the other hand, many instances have been observed where 
the effect of an earthquake has been to depress permanently the disturbed 
gi-ound. For example, by the Bengal earthquake of 1762, an area of 60 



PART I SECT, ii EARTHQUAKES 279 

square miles on the coast near Chittagong, suddenly went down beneath 
^he sea, leaving only the tops of the higher eminences above water. 
rhe succession of earthquakes which in the years 1811 and 1812 
levastated the basin of the Mississippi, gave rise to widespread depressions 
)f the ground, over some of which, above alluded to, the river spread so 
IS to form new lakes, with the tops of the trees still standing above the 
nirface of the water. 

Distribution of Earthquakes.^ — While no large space of the earth's 
(orface seems to be free from at least some degree of earthquake-move- 
nent, there are regions more especially liable to the visitation. As a 
•ule, earthquakes are most frequent in volcanic districts, the explosions 
)f a volcano being generally preceded or accompanied by tremors of 
preater or less intensity. In the Old World, a great belt of earthquake 
listurbance stretches in an east and west direction, along that tract of 
'emarkable depressions and elevations lying between the Alps and the 
nountains of northern Africa, and spreading eastward so as to enclose 
.he basins of the Mediterranean, Black Sea, Caspian, and Sea of Aral, 
kod to rise into the great mountain-ridges of Central Asia. In this zone 
ie numerous volcanic vents, both active and extinct or dormant, from the 
V^ores on the west to the basaltic plateaux of India on the east. The 
Pacific Ocean, surrounded with a vast ring of volcanic vents, has its 
)orders likewise subject to frequent earthquake shocks. Some of the 
nost terrible earthquakes within human experience have been those 
rhich have affected the western seaboard of South America. 2 It is worthy 
rf notice that the coasts of the Pacific Ocean more specially liable to con- 
nilsions of this nature plunge steeply down into deep water with slopes 
rf one in twenty to one in thirty, while shore -lines such as those of 
.^.ostralia, Scandinavia, and the east of South America, where the slope 
s no more than from one in fifty to one in two hundred and fifty, are 
aardly ever affected by earthquakes. It should also be remarked that 
^hile earthquakes are apt to occur along the flanks of mountain-chains and 

* For European earthquakes an alphabetical catalogue has been compiled bj' Professor 
D'Reilly, Trans. Roy. Inah Aatdemy, xxviii. (1886), p. 489. Catalogue of British earth- 
laakes, op. cit. xxviii. (1884), p. 285. C. Davidson, Geol. May. 1891, p. 450. Qxiart. Jour. 
Geci. Snc. xlvii. (1891), p. 618. Detailed observations of the effects of some recent European 
uirthqaakes will be found in the following Memoirs. The Andalusian earthquake of 25th 
Dec 1884, T. Taramelli and G. Mercalli, Real. Accad. Lincei, 1885-86, p. 116, Hebert, 
Campt. rend. 1885, Fouque, ihid. 20th April 1885, and the large quarto volume of reports 
by the mission specially sent to study the phenomena of this earthquake, MSmoircs Ac/ui. 
SeL 1889 ; the Ligurian earthquake of 23rd Feb. 1887, T. Taramelli and G. Mercalli, Ann. 
Uficw Vent rale Meteordoy.^ih'OiUnam. part iv. vol. viii. (1888), Real. Accail. Lincei, iv. 
[1888) ; the Agram earthquake of 9th Nov. 1880, *Grundzuge der Abyssodjiiamik, &c.,' 
by 8. Pilar, Agrara, 1881 ; the middle German earthquake of 6th March 1872, *Das 
Mitteldeutsche Erdbeben von 6 Marz 1872.' by K. von Seebach, Leipzig, 1873. See also the 
papers cited on pp. 270-273. 

' The Charleston Earthquake of 31st August 1886, has been fully discussed by Captain 
Dutton, yinth Ann. Rejtort V. S. Oeol. Survey, 1887-88, p. 209. The earthquakes of 
Central America are discussed by F. de Montessus de Ballore in a Memoir rewarded by the 
Acad. Sci. Nat. Saone et Loire, and published at Dijon, 1888. 



PART I SECT, iii UPHEAVAL AND DEPRESSION 281 

conceivably affect mountainous areas ; but we do not know how it 
would affect the sea-floor. In mountainous districts, many different 
degrees of shock, from mere tremors up to imporUmt earthquakes, have 
been observed, and these are not improbably due to sudden more or less 
extensive fractures of rocks still under great strain.^ Hoernes, from a 
study of European earthquake phenomena, concludes that though some 
minor earth-tremors may be due to the collapse of underground caverns, 
and others of local character to volcanic action, the greatest and most 
important earthquakes are the immediate consequences of the formation 
of mountains, and he connects the lines followed by earthquakes >nth the 
structural lines of mountain-axes.'-^ 

From what was stated at the beginning of the present section, it is 
evident that where the earth's crust in any region is in a critical condition 
of equilibrium, some connection may be exi)ected to l^e traceable between 
the frequency of earthquakes and the earth's p>osition with regard to the 
moon and sun, on the one hand, and changes of atmospheric conditions, 
on the other. ^ A comparison of the dates of recorded earthquakes seems 
to bear out the following conclusions; 1st. An earthijuake maximum 
occurs about the time of new moon ; 2nd. Another maximum appears 
two days after the first quarter .• 3rd. A diminution of activity occurs 
about the time of full moon ; 4th. The lowest earthquake minimum is on 
the day of the last quarter.^ There is likewise observable a seasonal 
maximum and minimum, earthquakes over most of the northern hemi- 
sphere occurring most frequently in winter, and least frequently in 
summer.'* Out of 656 earthquakes chronicled in France up to the year 
1845, three-fifths took place in the winter, and two-fifths in the summer 
months. In Switzerland they have been observed to be alx)ut three times 
more numerous in winter than in summer. The same fact is remarked 
in the history even of the slight earthquakes in Britain. A daily maxi- 
mum appears to occur about 2.30 A.M., and a minimum a>K)Ut three- 
quarters of an hour after noon. No connection has yet been satisfactorily 
established between the oecuiTence of earthquakes and sun-spots. The 
greater frequency of earth<iuake8 in winter might be exi)ected to indicate 
a relation between their occurrence and atmospheric pressure, and 
possibly earthquakes are more fre<iuent with a low than with a high 
barometer.^ 



Section iii. Secular Upheaval and Depression. 

Besides scarcely perceptible tremors and more or less violent move- 
ments due to earthquake -shocks, the crust of the earth is generally 

* See poateut p. 311. Suess, 'Entstehung der Alpen,* Vienna, 1875. 
' "Erdbeben Studien," Jahrb. Geof. RficJi.s. xxviii. (1878), p. 448. 

' J. F. J. Schmidt, "Studien iiber ErdbeWn," 2ud ed. (1879), p. 18. 

* Jln<i. p. 20. See the work.s of Perrey cite<l on p. 270. 

* Schmidt, op. cit. j), 23. F. Griiger, Seues Jahrh. 1878, p. 928. There does not 
appear to be any marked connection between the state of the barometer and the occurrence 
of earthquakes in Japan — J. Milne, * Earthquake^,' p. 268. 



282 DYXAMICAL GEOLOGY BOOKin 



believed to undergo in many places oscillations of an extremely quiet and 
uniform character, sometimes in an upm'ard, sometimes in a downward 
direction. So tranquil may these changes be, as to produce from day to 
day no appreciable alteration in the aspect of the ground affected, so that 
only after the lapse of several generations, and by means of careful 
measurements, can they reiilly be proved. Indeed, in the interior of a 
country nothing but a series of accurate levellings from some unmoved 
datum-line might detect the change of level, unless the effects of the 
terrestrial disturbance showed themselves in altering the drainaga Only 
along the sea-coast is a ready measure afforded of any such movement 

It is customary in p>opular language to speak of the sea rising or 
falling relatively to the land. We cannot conceive of any possible 
augmentation of the oceanic waters, nor of any diminution, save what 
may be due to the extremely slow processes of abstraction by the hydrar 
tion of minerals and absorption into the earth s interior. Any changes, 
therefore, in the relative levels of sea and land must be due to some re- 
adjustment in the form either of the solid globe or of its watery envelope 
or of l)oth. Playfair argued at the Iwginning of this century that no 
sulisidence of the sea-level could l)e local, but must extend over the globc.^ 
But it is now recognised that what is called the sea-level cannot possess the 
uniformity formerly attributed to it ; that on the contrary it must be liable 
to lociil distortion from the attractive influence of the land. Not only so, 
but the level of the surface of large inland sheets of water must be 
affected by the suiTounding high lands. 

Mr. R. S. AVoodward, whose lecent memoir on this subject has been 
cited (p. 35) has calculated that in a lake HO miles broad and 1000 
feet deep in the middle, the diflerence of level of the water-surface at 
the centre and at the margin may amount to between three and four 
feet.- As already stated he has further computed that the effect of the 
continents of Europe and Asia at the centre in disturbing the sea-level 
must amount to about 2900 feet, if we suppose that there is no deficiency 
of density underneath the continent, and to only about 10 feet if we 
suppose that the very existence of the continent implies such a deficiency.* 

Various suggestions have been made regiirding possible causes of 
alteration of the sea-level. (1) A shifting of the present distribution 
of density within the nucleus of the planet would aflect the position and 
level of the oceans (anfey p. 47). (2) As permanent snow and ice 
represent so much removed from the general Ixxly of water on the globe^ 
any large increase or diminution in the extent and thickness of the polar 
ice-caps must cause a corresponding variation in the sea-level {anky 
p. 20). (3) A change in the earth's centre of gravity, such as might 
result from the accumulation of large masses of snow and ice as an ice- 
cap at one of the poles, has been already referred to (p. 20) as tending to 

* ' Illustrations of the Huttoniau Theory/ 1802. The same conclusion was announced 
liv L. von Buch, * Reise durch Norwegen un<l Lapland,* 1810. 

- Bull. U. S. Geol. Surv. No. 48 (1888), p. 59. 

2 Op. i'U. p. 85. See Stokes, Traus. Camh. Phil. Srn:. \iii. (1849), p. 672 ; Sei, Proc. 
Roy. JtuUin S>c. v. (1887), p. 652. 



PART I SECT, iii UPHEA VAL AND DEPRESSION 283 

raise the level of the ocean in the hemisphere so affected, and to diminish 
it in a corresponding measure elsewhere. The return of the ice into the 
state of water would produce an opposite eftect. The atti-active influence 
of the ice-sheets of the Glacial Period upon the sea-level over the northern 
hemisphere has been discussed by various mathematicians, especially by 
Croll, Pratt, Heath, and Lord Kelvin. Considerable diff'erences appear in 
their results, according to the conditions which they postulate, but they 
agree that a decided elevation of the sea-level must be attributed to the 
accumulation of thick masses of snow and ice. The rise of the sea-level 
^ong the border of an ice-cap of SS'' angular radius and 10,000 feet 
thick in the centre is estimated at from 139 to 573 feet.^ (4) A still 
further conceivable source of geographical disturbance is to be found in 
the fact that, as a consequence of the diminution of centrifugal force 
owing to the retardation of the earth's rotation caused by the tidal wave, 
the sea-level must have a tendency to subside at the equator and rise 
at the poles. ^ A larger amount of land, however, need not ulti- 
mately be laid bare at the equator, for the change of level resulting 
from this cause would be so slow that, as Dr. Croll has pointed out, the 
general degradation of the surface of the land might keep pace with it 
and diminish the terrestrial area as much as the retreat of the ocean 
tended to increase it. The same writer has further suggested that the 
waste of the equatorial land, and the deposition of the detritus in higher 
latitudes, may still further counteract the effects of retardation and the 
consequent change of ocean-level. (5) Some geologists have supposed 
that where the earth's crust is loaded with thick deposits of sediment or 
massive ice-sheets it will tend to sink, while on the other hand denudation 
by unloading it promotes upheaval. 

The balance of evidence at present available seems adverse to any 
theory which would account for ancient and modern changes in the 
relative level of sea and land by variations in the figure of the oceanic 
envelope, save to a limited extent by the attraction caused by extensive 
masses of upraised land, and possibly in northern and southern latitudes 
by the attractive influence of large accumulations of snow and ice. Such 
changes are rather to be regarded as due to movements of the solid crust. 
The proofs of upheaval and subsidence, though sometimes obtainable from 

* See CroU, * Climate and Time,' chap, xxiii. xxiv. (MJ. Mwj. 1874. Pratt, ' Figure of the 
Earth.* D. D. Heath, PhU. Mag. xxxi. (1866), pp. 201, 323, xxxii. (1866), p. 34. Thomson 
(Lord Kelvin), op. cit. xxxi. p. 305. A. Penck, Jahrb. O'eofjrajjh. Gesel. Munich, vii. De 
Lapparent, Bull. Sot-, (t^tl. France^ xiv. (1886), p. 368, Revvc Ofn^rale des Sciences^ May 
1890. R. S. Woodward, Bull. U. S. Gf<^. Survey, No 48. Von Drygalski, ' Bewegungen 
der Kontinente zur Eiszeit,' Berlin, 1889. Prof. Suess believes that the limits of the dry land 
depend upon certain large indeterminate oscillations of the statical figure of the oceanic 
envelope ; that not only are "raised beaches" to be thus explained, but that there are 
absolutely no vertical movements of the crust save such as may form part of the plication 
arising from secular contraction ; and that the doctrine of secular fluctuations in the level 
of the continents is merely a remnant of the old "Erhebungstheorie," destined to speedy 
extinction. * Antlitz der Erde,' Leipzig, 1883. Pfaff defends the general opinion against 
these views in Zeitsch. Jteutsch. Ci*'d. Oes. 1884. 

' Croll, Phil. Mtnj. 1868, p. 382. Thomson, Trans. Gcol. Soc. Glasgow, iii. p. 223. 



284 DYNAMICAL GEOLOGY BOOKniPABTi 

^vide areas, are marked bv a want of uniformitv and a local and variable 
cbanicter, indicative of an action local and variable in its operations, such 
as the folding of the terrestrial crust, and not regular and iiddespread, 
such as might be predicated of any alteration of sea-level. While admit- 
ting therefore that, to a certjiin extent, oscillations of the relative level 
of sea and land may have arisen from some of the causes above enumer- 
ated, we may hold that, on the whole, it is the land which rises and sinks 
rather than the sea.^ 

§ 1. Upheaval. — Various maritime tracts of land have Ijeen asceitained 
to have undergone in recent times, or to be still undergoing, what appears 
to be a gradual elevation above the seji. On the coast of Siberia, for 600 
miles to the east of the river Lena, round the islands of Spitzbergen and 
Xovaja Zemlja, along the shores of the Scandinavian peninsula with the 
exception of a small area at its southern apex, and along a maritime strip 
of western South America, it has been proved that the sea stands now 
at a lower level with regard to the land than it formerly did. In 
searching for proofs of such movements tlie student must be on his guard 
against being deceived by any apparent retreat of the sea, which may be 
due merely to the dei)08it of gi'avel, sand, or mud along the shore, and 
the consequent gain of land. Local accumulations of gravel or " storm 
beaches," are often thrown up by storms, even above the level of ordinary 
high-tide mark. Li estuaries, also, considerable tracts of low ground are 
gradually raised above the tide-level by the slow deposit of mud. The 
following j)roofs of actual rise of the land are chiefly relied on by 
geologists. - 

Evidence from dead organisms. — Rocks covered with l>arnacles or other littoral 
adlierunt animals, or jtierced by lithodomous shells, aft'ord i»resimi[)tivo proof of the 
presence of the sea. A single stone with these creatures on its surface would not be 
satisfactory evidence, for it might have been cast up by a stonn ; but a line of large 
boulders, which had evidently not been moved since tlie cirripedes and mollusks lived upon 
tliem, and still more a solid clilf with these marks of littoral or sub-littoral life U]»on its 
base, now raistjd above high-water mark, would be sufficient to demonstrate a change of 
level. The amount of this change might be jjretty accuratel}' determined by measuring 
the vertical distance between the up|>er edge of the barnacle zone upon the upraised rock, 
and the limit of the same zone on the present shore. By this kind of eridence, the 
recent uprise of the coast of Scandinavia has been proved. The shell-borings on the 
pillai-s of the temple of Jupiter Serapis in the Bay of Naples prove first a depression and 
then an elevation of the ground to the extent of more than twenty feet.' Raised coral- 
reefs, forme<l by living sjiccies of corals, are a conspicuous feature of the geolog)* 
of the "West Indian Region. The teiraces of Barbadoes are jvirticularly striking. In 



' For the arguments against the view alx)ve adopted and in favour of the doctrine that 
the increase of the land above sea-level is due to the retirement of the sea, see H. Trast- 
KchoM, IhiJIetin Soci^tf Imp. ife^i yatvralistrs de Moscou, xlii. (1869), part i. p. 1 ; 188S, 
No. 2, p. 341 ; Bifll, .S'>r. Cteo/. France (3), viii. (1S7P), p. 134 ; but more especially Saess, 
in his great work the * Antlitz der Erde.' 

- See " Earthquakes and Volcanoes *' (A. G.), Cliambers's A[iscelhny of TracU. 

3 Babbage, E(Hii. Phil. Jovrn. xi. (1824), 91. J. D. Forbes. Edin. Joum. Sd, i. 
(1829), p. 260. Lyell, * Principles,' ii. p. 164. 



CT. iii § 1 EVIDENCE OF UPHEAVAL 285 



Ua, a raised coral-reef occurs at a height of 1000 or 1100 feet above the sea.* In Peni, 
dem coral- limestone has been found 2900 or 3000 feet above sea-level.''' Again, in 
J Solomon Islands, evidence of recent uprise is funiislied by coral reefs lying at a 
ght of 1100 feet,' and similar evidence occurs among the New Hebrides at 1500 feet. 
The elevation of the sea-lx)ttom can in like manner be proved by dead organisms 
ed in their (>osition of growth beneath high-water mark. Thus dead si)e«imens of 
ta trunaUa occur on some parts of the coast of the Firth of Forth in considerable 
mbers, still placed with their siphuncular end up])ermost in the stiff clay in which 
>y burrowed. The position of these shells is about high -water mark, but as their exist- 
; descendants do not live above low- water mark, we may infer that the coast has been 
9ed by at least the difference between high and low- water mark, or eighteen feet."* 
ad shells of the large Pholas dadyhis occur in a similar position near high-water mark 
the Ayrshire coast. Even below low-water, examples have been noted, as in the 
eresting case observed by Sars on the Drobaksbank in the Christiania Fjord, where 
d stems of OciUina proUfera (L. ) occur at depths of only ten or fifteen fathoms, 
is coral is really a deep-sea form, living on the western and northern coasts of Nor- 
jr, at depths of one hundred and fifty to three hundred fathoms in cold water. It 
st have been killed as the elevation of the area brought it up into upper and warmer 
ers of water. ^ It has even been said that the pines on the edges of the Norwegian 
•w-fields are dying in consetjuente of the secular elevation of the land bringing them 
into colder zones of the atmosphere. 

Any stratum of rock containing marine organisms which have manifestly lived and 
d where their remains now lie, may be held to prove a change of level between sea 
I land In this way it can be shown that most of the solid land now visible to us 
( once l>een under the sea. High on the flanks of mountain -chains (as in the Alps 
I Himalayas), undoubted marine shells occur in the solid rocks. 
Sea- worn Caves. — A line of sea- worn caves, now standing at a distance above 
:h- water mark beyond the reach of the sea, affords evidence of recent change of level. 
the accom])anying diagram (Fig. 75) examples of such caves are seen at the l^ase 
the cliff, once the sea-margin, now sei>arated from tlie tide by a platform of meadow- 
d. 

Raised Beaches furnish one of the most striking proofs of change of level. A 
-ch or space between tide-marks, where the sea is constantly grinding down sand and 
vel, mingling with them the remains of shells and other organisms, sometimes piling 
^ dei>osits up, sometimes sweeping them away out into oi)ener water, forms a familiar 
race or j»latform on coast-lines skirting tidal seas. When this margin of littoral 
>osits has Ikjcu i)laced above the reach of the waves, the flat terrace thus elevated is 
)wn as a ** raised beach " (Figs. 75, 76, 77, 78). The former high-water mark then lies 
lud, and while its sea-worn eaves are in time hung with ferns and mosses, the beach 
OSS which the tides once flowed furnishes a platfonn on which meadows, fields, 
dens, roads, houses, villages, and towns spring up, while a new beach ia made below the 
rgin of the uplifted one. A series of raised beaches may occur at various heights 
»ve the sea. Each terrace marks a fonner lower level of the land with regard to the 
, and probably a lengthened stay of the land at that level, while the intervals 
ween them represent the vertical amount of each variation in the relative levels of sea 
I land, and show that the interval lietween the changes was too brief for the forma- 
1 of terraces. A succession of raised beaches, rising above the j»resent sea -level, may 

^ A. Agassiz, Amer. Aciid. xi. (1882), p. 119. 
' A. Agassiz, Bull. Mus. Comp. Zool. vol. iii. 
' H. B. Guppy, Nature, 3rd January 1884. 

* Hugh Miller's ' Edinburgh and its Neighbourhood,' p. 110. 

* Quoted by Vom Rath in a paper entitled "Aus Norwegen," Neues Jahrb, 1869, p. 
i. For another exanii)le, see Owyn Jeffreys, Bn(, Assoc. 1867, p. 431. 



lyYNAAflCAL dEOLOGY 



BOOK UI PABt I 



therefore be lukeli as ]ioiiiting tn 
iiitprrupte<l liy lung [lauHcii, iliirhig 
unlesa in tegimis where there in renai 



11 riimier iiitvnnittcut nplipnvBl of the vountrj, 
L'hicli the funeral level did not niateriallj chuige, 
II to Wlieve that tlie Burfac« of the sea bu nndtr' 




accumulation or melting of large n 



.'» ud bvntail liy ( 



gone a cliaiige of level ivi: 
ice {ante, p. 20). 

Raised beaelied obonnd ill the higher latitiideit of the northern mid touthem heni- 
Hpheres, niid tliis di^tribntion huh bevti cluimed as a strong argument in favour of the 
view tlial tliey are due to ii fall of the touil level of the sva-HUrface from the djsa|>pear- 
ance or diminution of former iee-ca])^. Unit Hume at least of tlie rained beachea in thew 
regions may Iw due to this eiiiise niay l<e granteil. The grailual rise of level of the 
lieaches when traced ili> the Ijords. whieh has been 
i-ejienteilly asserted for sonic districts, would be the 
natural elfei't of the gi-pater uioaa of ii-e in the in' 
terior. In the exploration of the lake regions of 
Xorth America numerous instances iiave bren 
dewri1)ed of a slo]>e u|iv'ard of the former water- 
levels towards the main ice-liclds. A ramariuble 
example is fumiiihol by the terraces of the vanished 
glacial sheet ot Water called T^ke Agaraii which 
OUL1- tilled tint basin of the Ked River of the Korth. 
Mr. 1Vam-ii Upliam has found that these ancient 
lines of watci'-levpl gradually rise from south to 
, north and from west to en.st, in the direction of tlie 
I former ice-fields, the amount of olojie ranging irom 
1.1 blown «nJ (.()cffln|Hei«l bj^Uw i»ro to 1-3 feet |K!r mile,' Mr. O. K. Gilbert has 
noticed a rise of as iiinch as S feet in a mile among 
the old teiTocuH of I^ake Ontario.' 
r ivund many parts of the coast-line of Britain. De la Becbe 
!W (Fig. 77) of tt Corniah locality whcm the existing beach ii 
flanked by a ctitf of slate, b, continually cut away by the sea so that the overlying rwisvd 
iH'Och, a, r, nil! ere long disajijH'sr. The coast line on both sides of ScotUnd is likowiae 




ml Hull, Cornwall (D.) 
Rained beaches oct 
i the subjoined v 



' B«U. r. S. Oeol. Sun: Xo. i 



], pp. IS. 20. 



' .leiena, i. p. 221. 



EVIDENCE (IF UPHEAVAL 



friliged with raised beachea, sometimes four or live occun-itig above each other at heights 
of 25, 40, 50, eO, 75, and 100 feot aboTe the present high-water mark.' Others are found 
on both Bides of the Enghsh CI annrl The s lea f tt e ou ta nous tjonls of Is'ortlieni 




}Te thau 000 feet above sea-level, are marked with coiiapieuous litiee of 
These teiraees are partlj- ordinary l>each de[Hiait3, partly notches cut 




out of rock, probably with tlie aid of driftiug coast-ioe.' Pi™>f3 of recent elevation of 
the ahores of the Mediterranean are furnished by raised beaches at various heights alrave 



Die British raised benches, see De la Beche, * Report on Geology of 
lap. liii. ; C. Maclareu, ' Geology of Fife sud the Lothians,' 1838 ; 
Sea Maitfins ; ' Prestwieh, Q. J. Oed. Sue. xiviil. p. 38 ; mi. p. 
V. Holmea, Brit. Ataor. 1876, Seels, p. 95; Uasher, (iti-l. ilwj. 



' For icconnta of 
Devon and Cornwall,' 
R. Chambers, ' Anciet 
29 ; B. Russell and 1 

1878, p. 16fl. 

' On the raiaed beach of Sangntte. near Calais, see Preatwich, RvU. .Sor. GM. France 
(3), viii. (1880), p. 547 ; on those of Finisterre, C, Barrois, Ann. .foe. GM. Nord. ix. (188^). 

1 See R. Chambers, ' Trociugs of the North of Europe' (1850), p. 172 tl atq. Bravais, 
'Voyages de !a CommiasioQ Scientlfique du Nord,' tc, translated in C- ■!■ Qeol. Soc. L p. 
SS4. Kjemlf, Z. Dcutudi. Oeul. Ore. :txii. p. 1 ; 'Die Geologie des aud. und mittl. Nor- 
wagen,' 1880, p. 7 ; Otol. Mag. vjii. p. 74. S. A. Sexe, " On Rise of Land in Scandltuivia," 



288 /> YXAMR 'AL GEOLOG Y book in part i 



tin* present water-level. In Corsiea such terracesi occur at heights of from 15 to 20 
metres.* 

On the we.st eoast r»f South America, lines of raised terrace containing recent shells 
have heen traee<l by Daiwin as [irool's of a f^CAt upheaval of that i)art of the globe in 
moilern freolofjieal time. The terraces are not quite horizontal but rise towards tlw 
south. On the frontier of Bolivia, they weur at from 65 to 80 feet above the existing set- 
level, hut nearer the higher mass of the Chilian Andes they are found at 1000, and n«r 
Valj^iraiso at 130<) feet. That >ome of these ancient sea-margins l)elong to the humsn 
I«eri(Kl was shown by Mr. Darwin's distH»very of shells with bones of birds, ears of 
maize, plaited ivimIs and i-otton thread, in one of the terraces opposite Callao at a height 
<»f bo fe»*t.- Riiisetl Wai-hes (K-eur in Xew Zealand, and indicate a greater change of 
level in the southt'rn than in the northern [^ai-t of the country.' It should be obserred 
that this inci-easeil rise of the terrat.*es indewanls occurs both in the northern and 
southern hemisj>heivs. and is one of the chief facts insisted ujx)n by those who would 
exi'lain the ten•aee^ by dis[>lacements of the sea rather than of the land. 

Human Reconls and Traditions. — In countries which have been long 
si'ttlfd hy a human [lopulatiun. it is sometimes i^^ssihle to ]»rove, or at least to render 
prolwhle. the fact of i-ei-ent change of It'Vel hy reference to tradition, to local names, and 
to works of human construction. IMei-s and harlK>urs, if now found to stand above the 
u[»i>er limit of high-water, furnish indecMl indis]>utahle endencoof a rise of land or fidlof 
sca-levcl since their erection. Numerous pr«>ofs of a recent change of level in the coast of 
the Arctic Ocean from .S[»itzWrgen eastwanl have l»een observed. The Finnish coast is 
rcjtortetl to have risen tJ feet 4 inches in 127 yeai-s.* At SpitzWrgen itself, besides its 
rai^-d beacho. )»earing witne.ss to j»revious elevations, small islands which existed two 
hundrtnl yeai'> ago are now joine«l to larger i»ortions of land. At Novaja Zem\ja, where 
>ix rai>ed InMebes were foun<l by Xonlenskjold, the highest Wing 600 feet above set- 
level,'' theiv M'cms to have U'cn a rising of the sea-lx)ttom to the extent of 100 feet (W 
more since the Dutch e.\[K;dition of ir»iM. On the north coast of Siberia the island of 
Diomida. observe<l in 1760 byChalaourof to the east of Cape Sviatoj, was found by Wraugel 
-sixty years aft«rwan.ls to have l»een unitt^il to the mainland.^ From marks made on the 
c»K\>t in the middle of la>t century it ap[»ears that the north of Sweden has risen about 
7 feet in the last l.'»l yeai>. but that the movement has lessened southwards until in 
Srauia it ha^ l»een re]ilaced by ""ne in a downwanl tlirection .see p. 291). 

jj 2. Subsidence. — It is more ditticult to trace a downward movement 
of land. t»>r tht* evidence of each successive sea-margin is carried down 
and washe<l away or covered up. Tlie student >W11 take care to guanl 
himself apunst heing niishnl V>y mere proofs uf the advance of the sea on 

/.■./. .: .<;. .*/',■.'.„ -f' I'..:,' r<if'/, Ohri-ti-inia. 1S72. H. M.»hn. yt/t. Aftitj, Xaf. xxH, p. 1. 
D.ikyii^. fJ' '. M"t. 1^77. p. 7ii. K. IVtter-en, .I-./'. M".fh. Xot. rhriMinnin, 1878, p. 182, 
V. il^"-.'.' : *i- \ M '•!. 1*«7'J. I', ii*^ : Tr-ms-.i .\f.\-'"»s A,triihrthr, Til. 1880. Sitz, Akad. 
\V' , ::■ viii. 1-*^'.*. I.rlniiann. ' reV»er-eliemali;re Strandlinier, &c.," Halle, 1879 ; ^iisek. 
y. .. .V"* /-•<-. l-*^*:', p. 2>«». A. «J. H.i*:l»o!!i. ».'..-•. F'ii: FurhaihU, ikt>ckhiilm, ix. (1887), 
p. IV. r. Saii-llrr. /•■'.■-,/."«/?.< .\[:tth.i:. xxwi. tlS9u». pj.. 200. 2:35. 

' /;"7. .<■;. '.Vv. Fr.t,..-,' :V. iv. j.. <»;. 

- Geo^vi^^al i.>b^crvatioiis* chap. ix. r>ee ftV.7. J/,/./. Is77, p. 28. 

• Haa>t'.> M;t:«jloir>- «'lH"aiiicrbury.* 1>79. p. ot>»j. 

* y^t"... xxvi. i». 201. » //././. XV. jK 123. 

'^ i;ra.l. /;.'.■/. Sr, r;.',^. Fr.'n,.\ 3nl ser. ii. p. 04 S. Traces of oscillations of level 
>*itinn hi>roric time«i have JK-eu ob-iervevl in tlie Netherlands Flanders, and Upper Italy. 
/;•..". .ji'-. '.''/. F,''"h'. 2n.l ser. xix. p. .Xitf : :lr.l ser. ii. pp. 46, 222; -4«fi. Sk*e. GioL 
y /■■'. V. p. 'J IS. For aileu'ed cliauje> of level in the estuary of the Garonne, see 
\xfifmp. Ait. S..\ Linn. lk»nleaux. xxxi. (1^76 . p. 2S7, and Delfortrie, ib, xzxii. p. 79. 



BBCT. iii § 2 EVIDENCE OF SUBSIDENCE 289 

the land. In the great majority of cases, where such an advance is taking 
place, it is due not to subsidence of the land, but to erosion of the shores. 
It is, indeed, the converse of the deposition above mentioned (p. 284) as 
liable to be mistaken for proof of upheaval. The results of mere erosion 
by the sea, however, and those of actual depression of the level of the 
land, cannot always be distinguished without some care. The encroach- 
ment of the sea upon the land may involve the disappearance of suc- 
oeasive fields, roads, houses, villages, and even whole parishes, without 
any actual change of level of the land. Certain causes, however, referred 
to below, may come into operation, producing an actual submergence of land 
without any real subsidence of the land itself. The following kinds of 
evidence are usually cited to prove subsidence. 

Sabmerged Formats. ^As the land is brouftbt within reach of the iravea, and its 
elianctcnstic aurTace'featuree are effaced, the sabmerged area maj retain little or no 
erideTice of its having been a land-surface. It will be covered, as a rule, with sea-worn 
Mud or silt. Hence, no doubt, the reosoD v--\tf, among the marine strata which form 
■o much of the stratified portion of the earth's crast, and contain so many proofs of 
depreaaion, actual traces of land-BUiftcea are comparatively rare. It is only under very 
(avtmrable circumstances, as, for instance, where the ares is sheltered from prevalent 
winds and waves, and where, therefore, the sarface of the land can sink tranquilly under 
tlie sea, that Fragments of that surface ma; be preserved under overlying marine 
Mcnmulationa. It is in such places that "submerged forests " occur (Fig. 79). These are 



j|ifti>, ^Kd' ,\i^^ -. ■„ >S!iji^y-^ . I h jt , Ma 




■tamps of trees still in their positions of growth Jn their native soil, often associated 
with beds of peat, full of tree-roots, hazel-nuts, branches, leaves, and other indications 
of a terrestrial surface. Tliere is sometimes, however, considerable risk of deception in 
regard to the nature and value of such evidence of depression. Where, for instance, 
■bingia or sand is banked up against a nhore'or river-month, considerable spaces may be 
enclosed and tilled with frexb-water. the bottom of which may be some way below higb- 
water mark. In such lagoons terrestrial vegetation slid debris from the land may 
be deposited. Eventually, if the protecting barriers should be cut away the tides may 
Itoir over tbe layers of terrestrial |ieat, giving a false appearance of subsidence. Agalli, 
vwing to removal of subterranean aandy dejiosits by springs, overlying ^cat-beds may 
■ink below sea-level. 

De la Beche has described, round the shores of Devon, Cornwall, and westein 



290 DYNAMICAL GEOLOGY book m parti 

Somerset, a vegetable accumulation, consisting of plants of the same species as thon 
which now grow freely on the ac^oining land, and occurring as a bed at the mouths of 
valleys, at the bottoms of sheltered bays, and in front of and under low tracts of land, 
of which the seaward side dips beneath the present level of the sea.^ Over this sab* 
merged land -surface, sand and silt containing estuarine shells have generally been 
deposited, whence we may infer that, in the submergence, the valleys first became 
estuaries, and then sea-bays. If now, in the course of ages, a series of such submciyd 
forests should be formed one over the other, and if, finally, they should, by upheaval of 
the sea-bottom, be once more laid dry, so as to be capable of examination by boring, 
well-sinking, or otherwise, they would prove a former long-oontinued depression, witii 
intervals of rest. These intervals would be marked by the buried forests, and the progren 
of depression by the strata of sand and mud lying between them. In short, the evidenoe 
would be strictly on a i)arallel with tliat famished by a succession of raised beaches ai 
to a former protracted intermittent elevation. 

Along the coasts of Holland and the north of France^- submerged beds of peat have 
been regarded as proofs of submergence during historic times. The amount of change 
varies considerably in different places, and here and there can hardly be appreciated. 
The sinking during the 350 years preceding 1 850 is estimated to have amounted in the 
l)oldcrs of Groningen to a mean annual rate of 8 millimetres.^ In the north of Franee 
numerous examples of submerged forests have been observed. In 1846, in di gg ing the 
harbour of St. Servan, near St. Malo, a Gaulish cemetery containing ornaments and ooiiii, 
and resting on a still more ancient prehistoric cemetery, was met with at a level of 6 
metres below the level of high tide, so that the submergence must have been at least to 
that extent.' 

Coral-islands. — Evidence of widespread depression, over the area of the Faoifie 
and Indian Oceans, has been adduced from the structure and growth of coral-reefr and 
islands. Mr. Darwin, many years ago, stated his belief that, as the reef-building corals 
do not live at depths of more than 20 to 30 fathoms, and yet their reefs rise out of deep 
wat«r, the sites on which they have formed these structures must have subsided, the 
rate of subsidence being so slow that the upward growth of the reefs has on the whole 
kept i>ace with it.^ More recent researches, however, show that the phenomena of coral- 
reefs are in some cases, at least, capable of satisfactory explanation without subsidence, 
and hence that their existence can no longer be adduced by itself as a demonstratioii 
of the subsidence of large areas of the ocean. ^ The formation of coral-reefs is described in 
Book III. Part II. Section iii., and Mr. Darwin's theory is there more fully explained. 

Distribution of plants and animals. — Since the appearance of Edward Forbes*! 
essay upon the connection between the distribution of the existing fauna and flora of the 
British Isles, and the geological changes which have affected that area,^ much attention 
has been given to the evidence furnished by the geographical distribution of plants and 

1 "Geology of Devon and Cornwall," Mem. Geol. Survey. For further accounts of 
British submerged forests, see Q. J. Geol, Soc. xxii. p. 1. ; xxxiv. p. 447. OeaL Mag, vL 
p. 76 ; vii. p. 64 ; iii. 2nd ser. p. 491 : vi. pp. 80, 251. Mr. D. Pidgeon has aiguedin 
favour of the submerged forest of Torbay having been formed without subsidence of the land. 
Quart. Journ. Gtd. Soc xli. (1885), p. 9. See also W. Shone, op, cit xlviii. (1892), p. 96. 

' Lorii", Archives da Musie Teyler^ ser. ii. vol. iii. Part 5 (1890), p. 421. 

^ Lorie, ibid, p. 438, and papers cited postea, ]>. 292. But see Suess, ' Antlitz der Erde,* 
ii. p. 547. 

* See Darwin's 'Coral Islands,' Dana's 'Corals and Coral Islands,' and the works dted 
jtostea. Book III. Part II. Section iii. § 3, under "Coral-reefs " (p. 488). The various theories 
on the subject are discussed by K. Langenbeck iu his ' Theorien uber die Entstehung der 
Koralleninseln und Korollenriffe,* 1890. 

^ See Proc. Roy. Phys. Soc. Edinhurghy viii. p. 1. 

« Mem. Geol. Survey, vol i. 1846, p. 336. 



SECT, ui § 2 EVIDENCE OF SUBSIDENCE 291 

anim&ls as to geological revolutions. In some cases, the former existence of land now 
submerged lias been inferred with considerable confidence from the distribution of living 
organisms, although, as Mr. Wallace has shown in the case of the supposed *' Lemuria," 
some of the inferences have been unfounded and unnecessary.' The present distribution 
of plants and animals is only intelligible in the light of former geological changes. As 
a single illustration of the kind of reasoning from present zoological groupings as to 
former geological subsidence, reference may be made to the fact, that while the fishes 
and mollusks living in the seas on the two sides of the Isthmus of Panama are on the 
whole very distinct, a few shells and a large number of fishes are identical ; whence the 
inference has been drawn that though a broad water-channel originally separated North 
and South America in Miocene times, a series of elevations and subsidences has since 
oocurred, the most recent submersion having lasted but a short time, allowing the 
jiassage of locomotive fishes, yet not admitting of much change in the comparatively 
stationary mollusks.^ 

Fjords. — An interesting proof of an extensive depression of the north-west of Europe 
is famished by the fjords or sea-lochs by which that region is indented. A Qord is a 
long, narrow, and often singularly deep inlet of the sea, which terminates inland at the 
mouth of a glen or valley. The word is Norwegian, and in Norway Qords are character- 
istically developed. The English word " firth," however, is the same, and the western 
coasts of the British Isles furnish many excellent examples of Qords, such as the Scottish 
Loch Houm, Loch Nevis, Loch Fyne, Gareloch ; and the Irish Lough Foyle, Lough Swilly, 
Bantry Bay, Dunmanus Bay. Similar indentations abound on the west coast of British 
North America and of the South Island of New Zealand. Some of the Alpine lakes 
(Lucerne, Garda, Maggiore, and others), as well as many in Britain, are inland examples 
of Qords. 

There can be little doubt that, though now filled with salt water, Qords have been 
originally land-valleys. The long inlet was first excavated as a valley or glen. The 
adjacent valley exactly corresi^nds in form and character with the hollow of the Qord, 
and must be regarded as merely its inland prolongation. That the glens have been 
excavated by subaerial agents is a conclusion borne out by a great weight of evidence, 
which will be detailed in later parts of this volume. If, therefore, we admit the sub- 
aerial origin of the glen, we must also grant a similar origin to its seaward prolongation. 
Every Qord will thus mark the site of a submerged valley. This inference is confirmed 
by the fact that Qords do not, as a rule, occur singly, but, like glens on land, lie in grou^ts ; 
so that, when found intersecting a long line of coast, such as that of the west of Norway or 
the west of Scotland, they show that the sea now runs far up and fills submerged glens. 

Human constructions and historical records. — Should the sea be observed 
to rise to the level of roads and buildings which it never used to touch, should former 
half-tide rocks cease to be visible even at low water, and should rocks, previously above 
the reach of the highest tide, be turned first into shore- reefs, then into skerries and islets, 
we infer that the coast-line is sinking. Such kind of evidence is found in Scania, the 
most southerly part of Sweden. Streets, built of course above high-water mark, now 
lie below it, with older streets lying beneath them, so that the subsidence is of some 
antiquity. A stone, the position of which had been exactly determined by Linnaeus 
in 1749, was found after 87 years to be 100 feet nearer the water's edge.' The west 
coast of Greenland, for a space of more than 600 miles, is perceptibly sinking. It has 

' * Island Life,' 1880, p. 394. In this work the question of distribution in its geological 
relations is treated with admirable lucidity and fulness. 

• A. R. Wallace, ' Geographical Distribution of Animals,* i. pp. 40, 76. 

' According to Erdmann, the subsidence has now ceased, or has even been exchanged for 
an upward movement {Oeol. FOr. Stockholm Forhandl. i. p. 93). Nathorst also thinks that 
Scania is now sharing in the general elevation of Scandinavia {^id. p. 281). It appears that 
the zero of movement now passes through Bornholm and Laalaiid. 



292 DYXAMICAL GEOLOGY book in part I 

there been noticed that, over ancient buildings on low shores, as well as oyer entire 
islets, the sea has risen. The Mora\'ian settlers have been more than once driven to 
shift their boat -poles inland, some of the old poles remaining visible under water.^ 
Historical evidence likewise exists of the salwidence of ground in Holland ind 
Belgium.^ On the coast of Dalmatia, Roman roads and viUaa are said to be visible 
below the seA.^ 

§ 3. Causes of Upheaval and Depression of Land. — These move- 
ments must again be traced l)ack mainly to consequences of the internal 
heat of the earth. There are various ways in which this cause may have 
acted. As rocks expand when heated, and contract on cooling, we may 
suppose that, if the crust underneath a tract of land has its temperature 
slowly raised, as no doubt takes place round areas of nascent volcanoes, 
a gradual uprise of the gi'ound above will be the result. The gradual 
transference of the heat to another quarter may produce a steady subsidence. 
Basing on the calculations of Colonel Totten, cited on p. 299, Lyell 
estimated that a mass of red sandstone one mile thick, having its temperar 
tui'e augmented 200° Fahr., would raise the overlying rocks 10 feet, and 
that a portion of the earth's crust of similar character 50 miles thick, with 
an increase of 600' or 800'', might produce an elevation of 1000 or 1600 
feet^ But this computation, as Mr. Mellard Keade has pointed out^ 
takes account only of linear expansion. If from any cause the mass of 
rock whose temperature was augmented could not expand horizontally it 
would rise vertically, and unless some of the surplus volume could be 
disposed of by condensation of the rock, the uprise would be three times 
as much as the linear extension. Taking this view of the case, we find 
that a mass of the earth's crust twenty miles thick, heated lOOO'^ Fahr., 
and prevented from extending laterally, would rise 1650 feet.* 

Again, rocks expand by fusion and contract on solidification. Hence, 
by the alternate melting and solidifying of subterranean masses, upheaval 
and depression of the surface may possiblv be produced (see pp. 299, 
304). 

But evidently processes of this nature can only eflfect changes of level 
limited in amount and local in area. When we consider the wide tracts 
over which terrestrial movements are now taking place, or have occurred 

^ Tliese obsen'atious, which have been accM'.pted for at least a generation past (Prvc Oeol. 
S()C. ii. 1835, }>. 208), have recently been called in queatiou, but the alleged disproof is not 
convincing, and they are here retained as worthy of credence. See Suess, Verliand. Oeti. 
Ht'icJimnstaltf 1880, No. 11, and 'Antlitz der Enle,' ii. p. 415 ef seq, 

^ Besides the ]>a].>er of Lorie, quoted on p. 290, consult Lavaleye, ' Affaissement da sol et 
envasement des fleuves, sur\'enus dans les temps hi.stori(pies,' Brussels, 1859. Grad, BmR, 
Soc. Gktl, France^ ii. 3rd ser. p. 46. Arends, 'Physische Geschichte der Nordseektute,' 
1833. Com{)are also R. A. Peacock on ' Physical and Historical Evidences of vast Sinkings 
of land on the North and West Coasts of France/ &c., London, 1868. For submerged 
peat-beds on French coast, see A. Gas])ard, Ann, Sue, G6i)l, Xonl^ 1870-74, p. 40. On 
o.scillations of Frencli coast, T. Girard, Bull. Sue, G^njrnph, Paris, st-r. 6, vol. x. p. 225 : 
E. Delfortrie, Act. Soc. Linn. Bortleaux^ ser. 4, vol. i. p. 79. 

3 BoU, Cam, U*'ol, Italiano, 1874. p. 57. 

* * Principles,* ii. p. 235. 

* Melhird Reade, * Origin of Mountain Ranges' (1886), pp. 112, 114. 



SECT. iii§3 CAUSES OF UPHEAVAL AND SUBSIDENCE 293 

in past time, the explanation of them must manifestly be sought in some 
far more widespread and generally effective force in geological dynamics. 
It must be confessed, however, that no altogether satisfactory solution of 
the problem has yet been given, and that the subject still remains beset 
with many difficulties. 

Professor Darwin, in one of his memoirs already cited (ante, p. 21), 
has suggested a possible determining cause of the larger features of the 
Barth's surface. Assuming for his theory a certain degree of viscosity in 
the earth, he points out that, under the combined influence of rotation 
ind the moon's attraction, the polar regions tend to outstrip the equator, 
md to acquire a consequent slow motion from west to east relatively to 
t^he equator. The amount of distortion produced by this screwing motion 
tie finds to have been so slow, that 45,000,000 years ago, a point in lat. 
30** would have been 4 J', and a point in lat. 60" 14 J' further west, with 
reference to the equator, than they are at present. This slight transfer- 
3nce shows us, he remarks, that the amount of distortion of the surface 
strata from this cause must be exceedingly minute. But it is conceivable 
.hat, in earlier conditions of the planet, this screwing action of the earth 
may have had some influence in determining the surface features of the 
planet. In a body not perfectly homogeneous it might originate wrinkles 
it the surface running perpendicular to the direction of greatest pressure. 
^In the case of the earth, the wrinkles would run north and south at the 
Mjuator, and would bear away to the eastward in northerly and southerly 
atitudes, so that at the north pole the trend would be north-east, and at 
ihe south pole north-west. Also the intensity of the wrinkling force 
varies as the square of the cosine of the latitude, and is thus greatest at 
.he equator and zero at the poles. Any wrinkle, when once formed, 
rould have a tendency to turn slightly, so as to become more nearly east 
md west than it was when first made." 

According to the theory, the highest elevations of the earth's surface 
.hould be equatorial, and should have a general north and south trend, 
vhile in the northern hemisphere the main direction of the masses of 
and should bend round towards north-east, and in the opposite hemi- 
phere towards south-east. Prof. Darwin thinks that the general facts of 
errestrial geography tend to corroborate his theoreticjil views, though he 
dmits that some are very unfavourable to them. In the discussion of 
uch a theory, however, we must remember that the present mountain 
hains on the earth's surface are not aboriginal, but arose at many 
uccessive and widely-separated epochs. Now it is quite certain that the 
ounger mountain-chains (and these include the loftiest on the surface of 
he globe) arose, or at least received their chief upheaval, during the 
'ertiary periods — a comparatively late date in geological history. Unless 
re are to enlarge enormously the limits of time which physicists are 
filling to concede for the evolution of the whole of that history, we can 
ardly suppose that the elevation of the great mountain-chains took place 
t an epoch at all approaching an antiquity of 45,000,000 years. Yet, 
ccording to Prof. Darwin's showing, the supei-ficial effects of internal 
istortion must have been exceedingly minute during the past 45,000,000 



294 DYNAMICAL GEOLOGY BOOKmPAKri 

years. We must either therefore multiply enormously the periods re- 
quired for geological changes, or find some cause which could have 
elevated great mountain-chains at more recent intervals. 

But it is well Worth consideration whether the cause suggested hy 
Prof. Darwin may not have given their initial trend to the masses of land, 
so that any subsequent wrinkling of the terrestrial surface, due to any 
other cause, would bo apt to take place along the original lines. To be 
able to answer this question, it is necessary to ascertain the dominant line 
of strike of the older geological formations. But information on this 
subject is still scanty. In Western Eiu'ope, the prevalent line along 
which terrestrial plications took place during Palaeozoic time was certainly 
from S.W. or S.S.W. to N.E. or N.N.E., and the same direction is recog- 
nisable in the eastern Stat<;s of North America. But the trend of later 
formations is more varied. The striking contradictions between the 
actual direction of so many mountain-chains and masses of land, and 
what ought to be their line according to the theory, seem to indicate that 
while the effects of internal distortion may have given the first outlines 
to the land-areas of the globe, some other cause has been at work in later 
times, acting sometimes along the original lines, sometimes across them. 

The main cause to which geologists ;ire now disposed to refer the 
corrugations of the earth's suiiace is secular cooling and consequent con- 
traction.^ If our planet has been steadily losing heat by radiation into 
si)ace, it must have progressively diminished in volume. The cooling 
implies contraction. According to Mallet, the diameter of the earth is 
less by at least 189 miles since the time when the planet was a mass of 
liquid.^ But the conti-action has not manifested itself uniformly over the 
whole surface of the planet. The crust varies much in structure, in 
thermal resistance, and in the position of its isogeothcrmal lines. As the 
hotter nucleus contracts more rapidly by cooling than the cooled and 
hardened crust, the latter must sink down by its own weight, and in so 
doing requires to accommodate itself to a continually diminishing diameter. 
The descent of the crust gives rise to enormous tangential pressures. The 
rocks are crushed, cnimpled, and broken in many places. Subsidence must 
have been the general rule, but every subsidence would doubtless be 
accompanied with upheavals of a more limited kind. The direction of 
these upheaved tracts, whether determined, as Prof. Darwin suggests, by 
the effects of internal distortion, or by some original features in the 
structiu'e of the crust, would l>e apt to be linear. The lines, once taken 
as lines of weakness or relief from the intense strain, would probably be 
made use of again and again at successive paroxysms or more tranquil 
}>eriods of contraction. Mallet ingeniously connected these movements 
with the linear direction of mountain-chains, volcanic vents, and earth- 
quake shocks. If the initial trend to the land-masses were given as 
hypothetically stated by Prof. Darwin, we may conceive that after the 
outer parts of the globe had attained a considerable rigidity and coukl 

* For an able criticism of this view see Fisher's * Physics of Earth's Crust,* 2nd Edit 
Consult also Mr. Reade's 'Origin of Mountain Ranges.' 
- Phil. Trans. 1873, \\ 205. 



SECT. iii§3 CAUSES OF UPHEAVAL AND SUBSIDENCE 296 



then be only slightly influenced by internal distortion, the effects of 
continued secular contraction would be seen in the intermittent subsidence 
of the oceanic basins already existing, and in the successive crumpling and 
elevation of the intervening stiffened terrestrial ridges. 

This view, variously modified, has been Avidely accepted by geologists 
as furnishing an explanation of the origin of the upheavals and subsid- 
ences of which the earth's crust contains such a long record. But it is not 
unattended with objections. The difficulty of conceiving that a globe 
poesessing on the whole a rigidity equal to that of glass or steel could be 
corrugated as the crust of the earth has been, has led some writers to 
adopt the hypothesis already described {ante, p. 56), of an intermediate 
viscous layer between the solid crust and the solid nucleus, while others 
have suggested that the observed subsidence may have been caused, or at 
least aggravated, by the escape of vapours from volcanic orifices. But 
with modifications, the main cause of terrestrial movements is still sought 
in secular contraction. 

Some observers, following an original suggestion of Babbage,^ have 
supposed that upheaval and subsidence, together with the solidification, 
crystallization, and metamorphism of the layers of the earth's crust, may 
have been in large measure due to the deposition and removal of mineral 
matter on the surface. There can be no doubt that the lines of equal 
internal temperature (isogeothermal lines) for a considerable depth down- 
ward, follow approximately the contours of the surface, curving up and 
down as the surface rises into mountains or sinks into plains. The de- 
position of a thousand feet of rock will, of course, cause a corresponding 
rise in the isogeotherms, and if we assume the average rise of temperature 
to be 1° Fahr. for every 50 feet, then the temperature of the crust 
immediately below this deposited mass of rock will be raised 20°. But 
masses of sediment of much greater thickness have been laid down, and 
we may admit that a much greater increase of temperatiu'e than 20° has 
been effected by this means. On the other hand, the denudation of the 
land must lead to a depression of the isogeotherms, and a consequent 
cooling of the upper layers of the crust. 

It may be conceded that in so far as the internal structure of rocks 
may be modified by such progressive increase of temperature as would 
arise from superficial deposit, this cause of change must have a place in 
geological dynamics. But it has been urged that besides this effect, the 
removal of rock by denudation from one area and its accumulation upon 
another affects the equilibrium of the crust ; that the portions where de- 
nudation is active, being relieved of weight, rise, while those where 
deposition is prolonged, being on the contrary loaded, sink.^ This hypo- 
thesis has recently been strongly advocated by some of the geologists who 
have been exploring the Western Territories of America, and who point 

' Journ. Oeol. Soc. iii. (1834), p. 206. 

^ Similarly it has been contended that the accumulation of a massive ice-sheet on the 
land would cause a depression of the terrestrial surface. N. S. Shaler, Proc. Boston Nat. 
Hist. Soc. xvii. p. 288. T. F. Jamieson, Quart. Journ. Oeol. fihc. 1882, and Oeol. Mag. 
1882, pp. 400, 526. Fisher, * Physics of Earth's Cnist,' p. 223. 



296 JJYXAMICAL HEOLOGY book lU past I 

in proof of its truth to evidence of continuous subsidence in tracts where 
there was prolonged deposition, and of the uprise and curvature of 
originally horizontal strata over mountain ranges like the Uinta Mountaiiu 
in Wyoming and Utah, which have been for a long time out of water. 
To suppose, however, that the removal and deposit of a few thousand feet 
of rock should so seriously affect the equilibrium of the crust as to cause 
it to sink and rise in proportion, would e\ince such a mobility in the earth 
as could not fail to manifest itself in a far more powerful way under the 
influence of lunar and solar attraction. That there has always been the 
closest relation between upheaval and denudation on the one hand, and 
subsidence and deposition on the other, is undoubtedly true. But denuda- 
tion has been one of the consequences of upheaval, and deposition has 
been kept up only by continual subsidence. 

We are concerned in the present part of this volume only with the 
surface features of the land in so far as they bear on questions of geo- 
logical dynamics. The history of these features will be more conveniently 
treated in Book VIL after the structure and history of the crust have 
been described. Before quitting the subject, however, we may observe 
that the larger terrestrial features, such as the great ocean basins, the 
lines of submarine ridge surmounted here and there by islands chiefly of 
volcanic materials, the continental masses of land, and at least the cores 
of most great mountain chains, are in the main of high antiquity, stamped 
as it were from the earliest geological ages on the physiognomy of the 
globe, and that their present aspect has been the result not merely of 
original hypogene operations, but of long-continued superficial action by 
the epigene forces described in Book III. Part II. 



Section iv. Hypogene Causes of Changes in the Texture* 
Structure, and Composition of Rocks. 

The phenomena of hy|X)gene action considered in the foregoing pages 
relate almost wholly to the effects produced at the surface. It is evident, 
however, that these phenomena chiefly arise from movements within or 
beneath the earth's crust, and must be accom^mnied by very considerable 
internal changes in the rocks which form that crust These rocks^ 
subjected to enormous pressure, have been contorted, crumpled, and 
folded back u])on themselves, as if thousands of feet of solid limestones, 
sandstones, and shales had been merely a few layers of carpet ; they have 
been shattered and fractured ; they have in some places been pushed far 
above their original position, in others depressed far beneath it : so great 
has l>een the compression which they have imdergone that their com- 
ponent pai-ticles have in many places been rearranged, and even crystal- 
lized. They have here and there probably been reduced to actual fusion, 
and have been abundantly invaded by masses of molten rock from below. 

In the present section, the student is asked to consider chiefly the 
nature of the agencies by which such changes can be effected ; the results 
achieved, in so far as they constitute part of the architecture or structure 



8BCT. iv § 1 EFFECTS OF HEAT ON ROCKS 297 

of the earth's crusty will be discussed in Book IV. At the outset, it is 
evident that we can hardly hope to detect many of these processes of 
subterranean change actually in progress and watch their effects. The 
very vastness of some of them places them beyond our direct reach, and 
we can only reason regarding them from the changes which we see them 
to have produced. But a good number are of a kind which can in some 
measure be imitated in laboratories and furnaces. It is not requisite, 
therefore, to speculate wholly in the dark on this subject. Since the early 
and classic researches of Sir James Hall, great progress has been made in 
the investigation of hypogene processes by experiment. The conditions 
of nature have been imitated as closely as possible, and varied in different 
ways, with the result of giving us an increasingly clear insight into the 
I^ysics and chemistry of subterranean geological changes. The following 
pages are chiefly devoted to an illustration of the nature of hypogene 
action, in so far as that can be inferred from the results of actual experi- 
ment. The subject may be conveniently treated under three heads — 
1. The effects of mere heat ; 2. the influence of the co-operation of heated 
water ; 3. the effects of compression, tension, and fracture. 

§ 1. Effects of Heat. 

The importance of heat among the transformations of rocks has 
been fully admitted by geologists, since it used to be the watchword of 
the Huttonian or Vulcanist school at the end of last century. Three 
sources of subterranean heat may have at different times and in different 
degrees co-operated in the production of hypogene changes — the original 
Internal heat of the globe, the heat arising from chemical changes within 
the crust or beneath it, and the heat due to the transformation of mechanical 
energy in the crumpling, fracturing, and crushing of the rocks of the 
i^rust 

Rise of temperature by depression. — As stated above (p. 295), the 
mere recession of rocks from the surface owing to superposition of newer 
lepoeits upon them will cause the isogeotherms, or lines of equal sub- 
terranean temperature, to rise — in other words, >vill raise the temperature 
>f the masses so withdrawn. This can take place, however, to but a 
limited extent, unless combined with such depression of the crust as to 
idmit of thick sedimentary formations. From the rate of increment 
)f temperature downwards it is obvious that, at no great depth, the rocks 
must be at the temperature of boiling water, and that further down, but 
jtill at a distance which, relatively to the earth's radius, is small, they 
nay reach and exceed the temperatures at which they would fuse at the 
nirface. Mere descent to a great depth, however, will not necessarily 
"esult in any marked lithological change, as has been shown in the cases 
)f the Nova Scotian and South Welsh coal-fields, where sandstones, shales, 
;lays, and coal-seams can be proved to have been once depressed 14,000 
;o 17,000 feet below the sea-level, under an overlying mass of rock, and 
ret to have sustained no more serious alteration than the partial conversion 
)f the coal into anthracite. They have been kept for a long period 



298 DYNAMICAL GEOLOGY book m parti 

exposed to a temperature of at least 212'' Fahr. Such a temperature would 
have been sufficient to set some degree of internal change in progress, 
had any appreciable quantity of water been present, whence the absence 
of alteration may perhaps be exi)licablc on the supposition that these rocks 
were comparatively dry (p. 305). 

Rise of temperature by chemical transformation. — To what extent 
this cause of internal heat may be operative, forms part of an obscure 
problem. But that the access of water from the surface, and the con- 
sequent hydration of previously anhydrous minerals must produce local 
augmentation of temperature, cannot be doubted. The conversion of 
anhydrite into gypsum, which takes place rapidly in some mines, gives rise 
to an increase of volume of the substance (p. 345). Besides the remark- 
able manner in which the rock is torn asunder by minute clefts, crystals 
of bitter-spar and quartz are reduced to fragments.^ The amount of heat 
evolved during this process is capable of measurement The conversion 
of limestone into dolomite, on the other hand, which involves a diminution 
of volume, might likewise be made the subject of similar experimental 
inquiry. Experiments with various kinds of rocks, such as clay-elate, 
clay, and coal, show that when these substances are reduced to powder 
and mixed with water, they evolve hoat.- 

Rise of temperature by rock-crushing. — A fiu-ther store of heat 
is pro\4ded by the internal crushing of rocks during the collapse and 
re-adjustment of the cnist. The amount of heat so produced has been 
made the subject of direct experiment. Daubr^e has shown that, by the 
mutual friction of its parts, firm brick-clay can be heated in three-quarters 
of an hour from a temj)erature of 18^ to one of 40' C. (65° to 104° Fahr.)* 
The most elaborate and carefully conducted series of experiments yet 
made in this subject are those conducted by Mallet. He subjected 16 
varieties of stone (limestone, marble, porphyry, granite, and slate) in cubes 
averaging rather less than 1^ inches in height to pressures sufficient to 
crush them to fragments, and estimated the amount of pressure required, 
and of heat produced. The following examples may be selected from his 
table : * — 

* The microscopic structure of the stages in the couversion of anhydrite into g^'psom is 
described by F. Hamnierschuudt, Tschermak's Mineral. MitthirU, v. (1883), p. 272. 

- W. Skey, Ch^m. Xcws^ xxx. p. 290. 

*' 'Gi'ol. ExpiTinientale, ' p. 448 et seq. This distinguished chemist and geologist has 
during the last forty years devoted much time to researches designed to illostnte ex- 
j>erimentany the processes of geology. His numerous imix)rtant memoirs are scattered 
through the Annnles des Mines, Ctnnptes Rendus de VAauUmie, Bulletin de la SociUi 
Otologiqiie de Franre, and other ]>ublicationR. But he has collected and republished 
them as 'Etudes Synthrtiques de Geologic Expt'rimentale,' 8vo, 1879 — a storehouse of 
information. The admirable memoirs of Delesse in the same journals should also be studied. 
The transformation of aragonite into calcite has been shown by Favre and Silbermaim to 
give rise to a relatively large disengagement of heat. H. Le Chatelier, Compf, rend. (1893), 
p. 390. 

* Phil. Tram. 1873, p. 187. 



ECT. iv § 1 



SOURCES OF HEAT 



299 



Rock. 


Temperature 

(Fahr.) in 

1 cubic foot of 

rock due to work 

of crushing. 


Number of cubic 
feet of water at 
32 deg. evapo- 
rated Into steam 
at 212 deg. 


Volume of ice at 
82 deg. melted to 

water at 32 deg. 

by one volume of 

rock. 


Caen Stone, Oolite .... 
Sandstone, Ayre Hill, Yorkshire 
Slate, Conway .... 
Granite, Aberdeen .... 
Scotch furnace-clay porphyry . 
Rowley Rag (basalt) 


8^004 
47".79 
132^85 
155'-94 
198*.97 
213**.23 


0-0046 

0-0234 

0-07 

0-072 

0-083 

0-109 


0-04008 

0-2026 

0-596 

0-617 

0-724 

0-925 



Within the crust of the earth, there are abundant proofs of enormous 
tresses under which the rocks have been crushed. The weight of rock 
nvolved in these movements has often been that of masses several miles 
hick. We can conceive that the heat thus generated may have been 
ufBcient to promote many chemical and mineralogical rearrangements 
hrough the operation of water (postea, p. 305), and may even have been 
ere and there enough for the actual fusion of the rocks by the crushing 
f which it was produced. 

Rise of temperature by intrusion of erupted rock. — The great 
leat of lava, even when it has flowed out over the surface of the earth, 
as been already referred to, and some examples have been given of its 
fleets (pp. 226, 230). Where it does not reach the surface, but is injected 
nto subterranean rents and passages, it must effect considerable changes 
ipon the rocks with which it comes in contact. That such intruded 
^eous rocks have sometimes melted down portions of the crust in their 
lassage, can hardly be doubted. But probably still more extensive 
hanges may take place from the exceedingly slow rate of cooling of 
rupted masses, and the consequently vast period during which their 
leat is being conveyed through the adjacent rocks. Allusion vnH be 
aade in later pages to the observed amount of such " contact-meta- 
Qorphism " (p. 597 et seq). 

Expansion. — Rocks are dilated by heat. The extent to which this 
akes place has been measured with some precision for various kinds of 
ock, as shown in the subjoined table : — 



Rock. 



Linear expansion for 
every 1° Fahr. 



Black marble, Galway, Ire- \ , 
land . . . . / I 

Grey granite, Aberdeen 

Slate, Penrhvu, Wales . 

White marble, Sicily . 

Red sandstone, Poitland, 
Connecticut 



Authority. 



':} 



-00000247 = ;joiVin» 
-00000438 = 5yT,»5r^ 

-00000576 = rnWT 
•00000613 = iT,^»n7 

-00000953 = TTnVuir 



{Adie, Trans. Hoy. Soc. Edin. 
xiii. p. 366. 
Ihid. 
Ibid. 
Ibid. 
/Totten, Amer. Journ. Sci. 
\ xxii. (1832). 136.1 



* For additional results, see Mellard Reade's * Origin of Mountain Ranges ' (1886), 
. 109. 



300 DYXAMICAL GEOLOGY book in paw i 

According to these data, the expansion of ordinary rocks ranges from 
jilK)ut 2 4 7 to 9 '6 3 million ths for V Fahr. Even ordinary daily and 
seasonal changes of temperature suffice to produce considerable super- 
ficial changes in rocks (see p. 328). The much higher temperatures to 
which rocks are exposed by subsidence within the earth's crust must 
have far greater effects. Some experiments by Pfaff in heating from an 
ordinary tem|>erature up to a red heat, or abo